PRINCIPLES    OF    ANIMAL    HISTOLOGY 


THE  MACMILLAN  COMPANY 

NEW  YORK   •    BOSTON   •    CHICAGO 
ATLANTA  •    SAN   FRANCISCO 

MACMILLAN  &  CO.,  LIMITED 

LONDON   •    BOMBAY  •    CALCUTTA 
MELBOURNE 

THE  MACMILLAN  CO.  OF  CANADA,  LTD. 

TORONTO 


'  A  TEXT-BOOK 

OF   THE 

PRINCIPLES   OF   ANIMAL 
HISTOLOGY 


BY 
V 

ULRIC   DAHLGREN,    M.S. 

ASSISTANT   PROFESSOR   OF   BIOLOGY    IN   PRINCETON   UNIVERSITY 
AND 

WILLIAM   A.   KEENER,   A.B. 

ADJUNCT   PROFESSOR   OF   BIOLOGY   IN   THE   UNIVERSITY 
OF  VIRGINIA 


Nefo  gotfe 

THE   MACMILLAN   COMPANY 
1908 

All  rights  reserved 


COPYRIGHT,  1908, 
BY  THE  MACMILLAN  COMPANY. 


Set  up  and  electrotyped.     Published  June,  1908. 


Nortooot  tfrrss 

J.  8.  Cashing  Co.  —  Berwick  &  Smith  Co. 
Norwood,  Mass.,  U.S.A. 


WILLIAM    LIBBEY 

THROUGH   WHOSE   FRIENDLY   INTEREST 
THE  WRITERS   WERE    GIVEN   THE 

OPPORTUNITY 
TO   DO   THIS 


TABLE    OF   CONTENTS 

CHAPTER   I 

PAGE 

PROTOPLASM i 

CHAPTER   II 

THE  CELL 6 

CHAPTER  III 

MULTICELLULAR  ORGANIZATION:  PHYLOGENETIC 13 

CHAPTER  IV 

MULTICELLULAR  ORGANIZATION:  ONTOGENETIC 19 

CHAPTER  V 

A.  MITOSIS 23 

B.  AMITOSIS 37 

CHAPTER  VI 

A.  EPITHELIUM 42 

B.  THE  AMPLIFICATION  OF  EPITHELIAL  SURFACES 48 

C.  THE  ORIGIN  OF  GLANDS 52 

CHAPTER  VII 

A.  THE  SUPPORTING  AND  CONNECTING  TISSUES *      .  56 

B.  THE  SIMPLE  RIGID  FORMS  OF  SUPPORTING  AND  CONNECTING  TISSUES  57 

C.  THE  SIMPLE  TENSILE  FORMS  OF  SUPPORTING  AND  CONNECTING  TISSUES  61 

D.  THE  HIGHER  TENSILE  FORMS  OF  SUPPORTING  AND  CONNECTING  TISSUES  63 

E.  THE  HIGHER  RIGID  FORMS  OF  SUPPORTING  AND  CONNECTING  TISSUES  67 

F.  FATS 73 

CHAPTER  VIII 

A.  THE  TISSUES  OF  MOTION .        .        -76 

B.  THE  STRIATED  MUSCLE  TISSUES 81 

C.  THE  HISTOGENESIS  OF  STRIATED  MUSCLE 88 


viii  TABLE  OF  CONTENTS 

PAGE 

D.  HEART  MUSCLE 92 

E.  SMOOTH  MUSCLE  TISSUES 97 

F.  PECULIAR  FORMS  OF  MUSCLE 102 

CHAPTER   IX 

A.  THE  ELECTRIC  TISSUES 105 

B.  THE  ELECTRIC  TISSUES  OF  ELASMOBRANCH  FISHES       .        .        .        .108 

C.  HlSTOGENESIS   OF   THE   ELECTRIC   TISSUES   OF   ELASMOBRANCHS        .  113 

D    THE  ELECTRIC  TISSUES  OF  TELEOST  FISHES 115 

CHAPTER  X 

THE  TISSUES  OF  LIGHT  PRODUCTION 122 

CHAPTER  XI 

THE  TISSUES  OF  HEAT  PRODUCTION 141 

CHAPTER   XII 

A.  THE  CIRCULATORY  TISSUES  FOR  DISTRIBUTION  AND  COLLECTION         .  143 

B.  THE  CIRCULATORY  CHANNELS 149 

C.  THE  CIRCULATORY  MEDIA 162 

D.  THE  BLOOD-FORMING  GLANDS 166 

CHAPTER   XIII 

A.  THE  NERVE  TISSUES:   THE  NEURON 174 

B.  THE  NERVE  CELL 179 

C.  THE  NERVE  FIBER 187 

D.  THE  MOTOR  NERVE  ENDING 191 

E.  NEUROGLIA  AND  GANGLION  STRUCTURE 196 

F.  THE  TISSUES  OF  TOUCH  OR  TACTILE  TISSUES 200 

G.  THE  TISSUES  OF  EQUILIBRATION  OR  STATIC  TISSUES    ....  207 
H.  THE  TISSUES  OF  HEARING  OR  AUDITORY  TISSUES         .        .        .        .215 

I.    THE  TISSUES  OF  SIGHT  OR  VISUAL  TISSUES 224 

J.    THE  TISSUES  OF  TASTE  AND  SMELL  OR  GUSTATORY  AND  OLFACTORY 

TISSUES 258 

CHAPTER   XIV 

PIGMENT  TISSUES 269 

CHAPTER   XV 

A.  THE  ALIMENTARY  TISSUES:   INTRA-CELLULAR  FORMS  OF  DIGESTION    .  279 

B.  MASTICATION 287 


TABLE  OF  CONTENTS  ix 

PAGE 

C.   PANCREATIC,  GASTRIC,  HEPATIC,  AND  ACCESSORY  FORMS  OF  DIGES- 
TIVE TISSUES       ...........    297 

CHAPTER   XVI 
THE  DUCTLESS  GLANDS 304 

CHAPTER  XVII 

A.  THE  TISSUES  OF  RESPIRATION 319 

B.  AIR-BREATHING  TISSUES  OR  LUNGS 321 

C.  WATER-BREATHING  TISSUES  OR  GILLS 326 

CHAPTER  XVIII 
THE  GAS-SECRETING  TISSUES  OF  ANIMALS 333 

CHAPTER    XIX 
THE  EXCRETORY  OR  NEPHRIDIAL  TISSUES 339 

CHAPTER   XX 

A.  INTEGUMENT,  MECHANICAL  PROTECTION -358 

B.  OFFENSIVE    MECHANICAL    PROTECTION    AND    THE    PRODUCTION     OF 

POISONOUS  FLUIDS 375 

C.  TISSUES  THAT  PRODUCE  LUBRICATING  FLUIDS         .....  387 

D.  TISSUES  THAT  PRODUCE  ATTRACTIVE  AND  REPULSIVE  ODORS       .        .  4OO 

E.  TISSUES  OF  ADHESION  AND  SPINNING 409 

CHAPTER  XXI 

A.  TISSUES  OF  REPRODUCTION 418 

B.  MALE  REPRODUCTIVE  CELLS  AND  NURSE  CELLS  OF  THE  SPERMATO- 

ZOON      423 

C.  FEMALE  REPRODUCTIVE  CELLS  AND  NURSE  CELLS  OF  THE  OVUM         .    453 

CHAPTER  XXII 
NlDAMENTAL  TISSUES,  USED   TO  FORM   COVERINGS  FOR    THE  OVA*  AND 

SPERMATOZOA 478 

ERECTILE  TISSUES 490 

CHAPTER  XXIII 
NOURISHING  MEMBRANES  AND  TISSUES  OF  THE  PARENT  AND  OF  THE  YOUNG    492 

CHAPTER   XXIV 
TECHNIC .        .502 

INDEX  .         .         ... 509 


INTRODUCTION 

A  TEXT- BOOK  of  histology  must  have  some  very  definite  reason  for 
appearing  at  the  present  time  when  so  many  good  books  bearing  that 
title  have  appeared  in  the  last  few  years  and  promise  to  continue  to 
appear.  These  books,  however,  with  practically  no  exceptions,  have 
been  intended  for  the  medical  school,  and  with  this  in  view  have  been 
restricted  to  the  histology  of  man,  as  a  main  theme,  with  more  or  less 
reference  to  other  animals,  principally  the  mammals. 

Such  works  have  not  filled  the  need  of  a  text-book  of  general  histology 
for  the  college  course,  a  book  that  treats  of  the  principles  of  the  subject. 
They  do  not  show  what  the  animal  cell  is  capable  of  as  a  builder  of  tissues 
which  enable  the  organism  to  make  use,  more  or  less  completely,  of  nearly 
all  the  known  forces  of  physics  and  many  of  those  of  chemistry.  This 
can  be  realized  when  we  consider  that  they  do  not  even  mention  such 
tissues  as  produce  electricity,  light,  gases,  and  many  other  things.  And 
again  much  is  lost  in  the  college  course  in  histology  by  the  fact  that,  of 
those  tissues  which  are  treated  of  in  the  medical  histology,  the  complete 
significance  is  lost  by  not  seeing  their  earlier  representation  and  variation 
in  the  lower  forms. 

This  volume  has  been  written  to  secure  a  work  that  covers  the  general 
field  of  histology  and  is  not  restricted  in  the  main  to  human  and  mamma- 
lian forms.  It  is  intended  to  be  a  work  that  teaches  general  principles 
and  teaches  histology  as  a  pure  science  and  for  its  own  sake.  It  is 
believed  that  it  will  serve  as  a  broad  foundation  for  future  studies  of 
morphology  and  embryology  as  well  as  for  the  medical  studies. 

As  to  method  of  treatment,  it  has  seemed  convenient  to  treat  each 
part,  with  some  exceptions,  by  writing  a  general  discussion  of  the  subject, 
and  following  this  discussion  with  detailed  laboratory  descriptions  of 
types  of  the  tissue,  abundantly  illustrated  with  good  pen  and  ink  drawings. 
Such  concrete  examples  have  been  selected  from  readily  accessible 
materials  in  most  cases,  and  the  proper  procedure  for  securing  and  pre- 
paring the  materials  has  been  indicated  in  small  type  at  the  end  of  each 
chapter  in  many  cases.  In  the  last  part  of  the  book  a  chapter  on  technique 
will  be  found,  giving  a  short  statement  of  principles  and  a  guide  for  some 
concrete  practice.  In  some  chapters  the  seminar  work  is  not  separated 
from  the  statements  of  principles.  Many  instructors  will  see  the  ad- 


xii  INTRODUCTION 

vantage  of  giving  the  student  closely  allied  but  different  examples  to 
work  out  with  these  descriptions. 

The  work  is  primarily  a  treatise  on  animal  histology.  Yet  some  of  it 
is  due  to  researches  that  have  been  carried  out  on  plant  tissues.  In 
several  places  fundamental  facts  are  illustrated  by  materials  taken  from 
the  vegetable  kingdom. 

The  arrangement  and  order  of  presentation  of  the  matter  have  been 
given  much  thought.  It  has  been  suggested  by  many  that  the  arrange- 
ment have  some  relation  to  the  onfogenetic  development  of  the  animal 
body  and  that  the  classification  of  the  tissues  be  based  upon  their  origin 
from  the  dividing  oosperm.  While  the  many  advantages  of  such  a  treat- 
ment have  been  fully  realized,  the  writers  have  felt  that  a  really  fairer 
and  more  logical  method  would  be  an  arrangement  on  a  basis  of  function 
in  the  first  place  with  ontogenetic  and  a  possible  phylogenetic  origin 
treated  of  necessity  in  the  second  place.  We  believe  that  such  a  direct 
treatment  is  not  only  more  convenient  and  clear,  but  that  it  is  more  logi- 
cal and  true,  and  that  it  will  serve  the  better  to  correlate  the  student's 
conceptions  concerning  homology  and  analogy  and  kindred  relationships. 

Perhaps  the  principal  practical  objection  to  the  use  of  the  embryo- 
logical  idea  in  the  arrangement  of  this  volume  would  be  the  great  differ- 
ences that  exist  in  the  fundamental  facts  of  cell-lineage  and  histogenesis 
among  the  principal  groups  of  animals.  So  long  as  the  work  dealt  with 
the  vertebrates  alone,  this  difficulty  would  not  emerge,  but  in  a  really 
broad  treatment  on  this  basis  of  almost  any  principal  tissue,  much  use- 
less repetition  and  lack  of  unity  would  be  encountered.  The  great  value 
of  treating  the  subject  from  the  view  point  of  function  can  better  be  under- 
stood when  it  is  remembered  that  all  structures  exist  only  for  the  purpose 
of  performing  certain  functions  in  some  particular  way. 

The  embryological  method  has  not  been  slighted,  however,  in  arrang- 
ing this  course.  It  either  parallels,  or  even  supplants,  at  a  number  of 
points,  the  arrangement  we  have  adopted,  and  it  is  given  a  fair  if  second- 
ary exposition  throughout  the  work.  This,  as  well  as  other  arrangement, 
has  necessitated  some  small  repetition  of  certain  ideas.  The  writers  have 
not  hesitated  to  repeat  leading  ideas  when  it  was  needful  and  possible,  at 
the  same  time,  to  look  at  them  from  different  points  of  view. 

A  full  bibliography  has  not  been  inserted  in  consideration  of  the  fact 
that  the  book  is  intended  for  college  students,  and  it  was  thought  that 
a  long  list  of  articles,  some  of  them  old  and  others  inaccessible,  would 
tend  to  discourage  any  further  reading.  From  one  to  three  broad  and 
modern  articles  by  recognized  authorities  have  been  mentioned  after 
each  part,  together  with  the  best  general  reference  books.  If  the  student 
is  to  go  farther  into  the  subject,  the  instructor  should  accustom  him  to 
looking  up  references  in  the  "  Zoologischer  Jahresbericht"  of  Naples, 


INTRODUCTION  xiii 

the  Journal  of  the  Royal  Microscopical  Society,  and  "Schwalbes 
Jahresbericht "  of  Heidelberg,  as  well  as  in  the  Zurich  Index  or  Zoo- 
logical Record.  He  should  also  be  instructed  how  to  find  the  latest  work 
on  a  subject  that  he  is  seriously  studying  or  is  carrying  out  research 
work  upon.  Also  to  find  out  who  is  doing  such  work  or  is  interested 
in  it. 

The  book  does  not  pretend  to  stand  as  an  authority  or  court  of  last 
resort  for  the  specialist  in  matters  of  general  histology.  'In  most  cases 
it  does  not  carry  the  student  into  debated  ground.  It  does  attempt 
to  be  a  convenient  teaching  guide,  to  gain  the  interest  of  its  readers  and 
to  stimulate  original  thought.  It  is  based  on  the  course  in  "  General 
Histology"  given  in  Princeton  University  by  the  senior  author  during 
the  last  ten  years. 

The  writers  have  many  to  thank  for  favors  and  help  in  connection 
with  the  work.  Our  colleagues  in  both  universities  have  often  aided  us. 
Dr.  H.  E.  Jordan  has  made  drawings  for,  revised,  and  written  parts  of 
the  work  on  the  reproductive  tissues.  Professors  A.  H.  Tuttle  and 
Wm.  Libbey  have  read  part  of  the  text. 

We  wish  to  thank  the  United  States  National  Museum  and  the 
United  States  Fish  Commission  for  some  rare  materials  given  through 
the  courtesy  of  their  officials.  Also  the  Carnegie  Institution  for  much 
material  collected  at  the  Tortuges  Laboratory  through  the  kindness 
of  Dr.  A.  G.  Mayer.  The  writers  are  most  appreciative  as  well  as  proud 
of  these  American  institutions,  which  are  of  such  continued  aid  to  science. 

Mr.  and  Mrs.  Alfred  Mitchell  most  generously  provided  the  oppor- 
tunity to  collect  valuable  material  in  Jamaica,  which  is  rich  in  many  rare 
forms.  Dr.  F.  R.  Lillie  of  Chicago  sent  us  Unio  material  at  a  time  of 
the  year  when  it  could  not  be  collected,  and  the  Yale  Museum  furnished 
us  with  some  rare  Schizopods. 

We  wish  to  extend  special  thanks  to  Mr.  Charles  A.  McAlpin  of 
Princeton  for  his  great  help  in  providing  the  Zoological  department  of 
the  university  with  quantities  of  new  and  old  books,  back  files  of  periodi- 
cals, and  other  literature.  His  generosity  is  one  of  the  factors  which 
has  made  it  possible  to  do  this  work  in  Princeton. 


HISTOLOGY 

CHAPTER    I 
PROTOPLASM 

AN  outline  of  the  discoveries  which  have  led  us  to  our  present  views 
of  life  phenomena  would  not  have  the  logical  sequence  of  the  method  by 
which  these  same  facts  can  be  unfolded  to  the  student  of  to-day.  Know- 
ing first  only  the  axiom  that  living  things  lived,  attention  was  first  drawn 
to  the  hollow  cell-like  structure  of  plants;  and  animals  were  supposed 
to  be  likewise  constructed.  Following  this,  we  came  to  know  the  more 
important  fact  that  a  semifluid  content  of  these  cells  was  a  specific  sub- 
stance and  was  the  fundamental  life-material  in  living  beings.  Proto- 
plasm was  the  name  given  this  substance. 

Working  forward  again,  we  learned  of  the  organization  of  this  pro- 
toplasm into  the  protoplast  or  solid  living  occupant  of  Schleiden  and 
Schwann's  "cell,"  with  which  its  name  is  so  hopelessly  entangled  that 
we  must  drop  the  better  term  here  and  use  the  hollow  title  of  "cell" 
for  the  life  unit  from  this  point  on. 

Science  was  now  free  to  branch  out  into  many  wide  fields  and  with 
much  correction  of  error  and  understanding  of  truth  to  build  up  our 
present  accumulation  of  facts  which,  when  compared  with  the  great 
truths  we  feel  must  really  exist,  is  very  small.  That  these  truths  hinge 
primarily  upon  protoplasm,  and  consequently  upon  the  cell,  the  tissue, 
the  organ,  and  lastly  upon  the  individual,  gives  us  an  assurance  of  the 
fundamental  character  and  importance  of  our  study  of  histology  which 
deals  with  the  first  three  of  these  fields. 

Protoplasm,  then,  is  our  beginning,  and  we  find  first  that  it  is  present 
in  all  life  forms  and  that  the  life  manifestations  occur  through  the  working 
of  its  substance  only.  It  is  a  transparent,  viscid  material  to  the  sight  and 
touch  and  makes  up  a  considerable  part  of  a  plant  or  animal  body;  not 
all  necessarily,  as  more  or  less  of  such  a  body  is  built  up  of  other  and 
non-living  substances,  which,  however,  are  controlled  entirely  by  the 
living  protoplasm. 

Of  next  greatest  interest  is  perhaps  the  question,  What  is  protoplasm 
composed  of  chemically;  and  how  is  it  put  together  structurally?  A 
qualitative  analysis  shows  us  that  four  out  of  the  many  known  elements 


2  HISTOLOGY 

of  our  universe  are  invariably  mostly  concerned,  —  carbon,  oxygen, 
hydrogen,  and  nitrogen;  and  that  secondly,  smaller  quantities  of  sul- 
phur, phosphorus,  iron,  etc.,  are  found,  some  of  them  only  occasionally, 
in  its  mass.  The  four  main  elements  occur  in  apparently  constant  quan- 
tities: carbon  45  parts;  oxygen  28  parts;  hydrogen  8  parts;  nitrogen 
15  parts;  and  phosphorus  4  parts,  which  leads  one  to  suppose  them  united 


FIG.  i.  —  Ovarian  ovum  of  a  cat  just  before  maturity,     c.m.,  cell  membrane;  n.m,  nuclear 
membrane;  nd.,  nucleolus;  mics.,  microsomes;  yk.al.,  yolk  alveoli. 


in  a  chemical  formation  in  which  the  second  group  of  elements  apparently 
do  not  take  direct  part  except,  perhaps,  the  phosphorus. 

Turning  for  light  to  the  structure,  it  is  seen  that  the  protoplasm 
in  a  given  protoplast  or  cell  is  not  a  homogeneous  mass;  it  is  differen- 
tiated into  several  organs  of  a  considerable  size,  as  a  nucleus,  centrosome, 
plastid,  nucleolus,  etc.  (Fig.  i),  while  in  finer  structure  it  is  evidently 
composed  of  several,  at  least  two  or  three,  substances  arranged  in  some 
sort  of  a  tissue  of  threads,  network,  or  alveolus,  mixed  with  granules. 
This  finer  structure  has  been  described  by  many  investigators  as  either 


PROTOPLASM 


FIG.  2.  —  Diagram  to  illustrate  the  alveolar  theory 
of  protoplasm. 


alveolar  like  a  mass  of  foam  (Fig.  2)  or  reticular  like  a  sponge  (Fig.  3). 
In  either  case  this  assumes  the  presence  of  at  least  two  substances  of 
different  qualities.  Among  these  qualities  it  can  be  perceived  that  one 
substance  is  of  denser  and  firmer  structure  than  the  other,  which  is  fluid. 
This  denser  substance,  called  .  ^_^^^^^ 

spongioplasm,  forms  the  matrix 
according  to  the  alveolar  the- 
ory, or  the  reticulum  according 
to  the  reticular  theory.  Thus  it 
is  a  continuous  and  communi- 
cating mass  in  both  cases,  while 
the  fluid  material  called  the 
paraplasm  is  considered  to  be  a 
continuous  and  communicating 
mass  in  the  reticular  theory 
only,  and  is  isolated  in  bubble- 
like  portions  according  to  the 
alveolar  theory.  Granules  of 
a  much  denser  material  are  found  scattered  about  in  the  reticulum 
or  matrix.  They  are  called  microsomes.  The  alveolar  theory  is 
the  more  probable,  at  the  same  time  admitting  that  in  many  cases 
and  at  certain  times  a  thread  formation  of  more  or  less  extent  does 
exist,  its  threads  lying  in  the  inclosing  substance  of  the  alveolar 
matrix. 

Knowing,  then,  that  protoplasm  is  differentiated  structurally  in  the 
cell,  we  are  prepared  to  hear  from  the  chemists  again,  who  tell  us  that 

protoplasm  is  not  a  definite 
chemical  entity,  but  a  combina- 
tion of  several  chemical  com- 
pounds united  in  a  physiological 
alliance,  and  interacting  on  one 
another  in  such  a  manner  as  to 
produce  the  phenomena  which 
we  take  as  evidence  of  existing 
life.  Passing  by  some  of  those 
substances  whose  position  is 
doubtful,  we  can  say  with  prob- 

ability    that    those    of    the    COm- 

pounds  known  as  the  proteids 
are  the  principal  figures  in  this  alliance  and  the  seat  of  real  life.  Though 
composed  of  nearly  the  same  materials,  they  are  of  very  great  variety, 
due  to  differences  in  the  method  of  composition  of  these  materials. 
Several  of  them  are  usually  concerned  in  the  structure  of  any  given 


FIG.  3.  —  Diagram  to  illustrate  the  reticular  theory 
of  protoplasm. 


4  HISTOLOGY 

cell,  while  different  cells  belonging  to  different  forms  of  life  have  each 
their  own  peculiar  proteid  or  proteids. 

When  we  realize  that  these  proteids  are  very  changeable  and  that  they 
go  through  extensive  change  upon  death,  we  can  understand  the  diffi- 
culty of  their  study  and  why  we  know  so  little  about  them. 

The  questions  concerning  the  exact  nature  of  life  lie  bound  up  in  pro- 
toplasm, and  but  one  of  them  will  be  commented  upon  here.  Behind 
these  phenomena  of  life  is  there  any  law  or  principle  other  than  the 
present  known  laws  of  chemistry  and  physics  that  is  responsible  for  the 
manifestations  of  the  life  phenomena  ?  It  is  our  belief  that  there  is,  for 
in  no  other  way  can  we  conceive  of  the  maintenance  of  so  many  delicate 
and  variable  phenomena  for  so  long  a  period  through  so  large  a  number 
of  different  conditions. 

That  many  of  the  life  phenomena  can  be  shown  to  have  a  direct 
sequence  to  some  chemical  or  physical  conditions  does  not  convince 
that  such  chemical  and  physical  conditions  are  the  first  cause  of  the 
phenomena ;  and  the  fact  that  life  is  only  maintained  within  certain  chemi- 
cal and  physical  limits  and  conditions  again  does  not  show  that  even  such 
conditions  are  capable  of  maintaining  it  for  even  a  short  time.  The 
mathematical  law  of  probability  and  chance  would  alone  convince  one 
of  the  futility  of  trying  to  make  matter  live  for  an  instant  upon  such  a 
basis,  when  so  many  conditions  are  constantly  interacting  and  dependent 
upon  one  another.  As  to  the  nature  or  origin  of  such  a  "vital"  law  or 
set  of  fixed  principles,  we  know  nothing.  That  such  a  law  or  laws  are 
supernatural  we  deny  on  the  ground  that  any  law  once  established  and 
continued  with  the  probability  of  permanence  in  nature  is  as  natural 
as  any  of  the  laws  we  know  about.-  That  this  unknown  "vital"  law  is 
permanent  and  is  subject  to  rigid  continuance  without  lapse  or  excep- 
tion is  the  only  ground  on  which  it  can  be  discussed.  We  have  not  even 
learned  enough  of  its  manifestations,  as  yet,  to  in  any  way  define  it  or  to 
classify  its  results  other  than  to  believe  that  it  directs  and  controls  the 
life  processes  to  ends  which  vary  only  with  the  circumstances  under 
which  they  exist. 

Protoplasm  is  protoplasm  only  so  long  as  it  is  living.  As  before 
mentioned,  life  must  be  regarded  as  a  process  that  is  taking  place  con- 
stantly in  the  protoplasm.  It  is  a  complicated  process  which  results  in 
constant  change  and  requires  a  constant  supply  of  new  material  or  food. 
As  a  result  of  this  activity,  energy  is  produced  in  the  form  of  motion, 
heat,  light,  and  electricity;  also  many  materials  are  elaborated,  as  acids, 
carbohydrates,  digestive  fluids,  protective  and  supporting  materials, 
poisonous  and  offensive  substances,  etc.,  which  are  necessary  to  the  exist- 
ence of  the  creature  to  which  the  protoplasm  belongs. 

The  dynamic  products  of  protoplasm — heat,  light,  electricity,  and 


PROTOPLASM  5 

motion  —  are  not  produced  directly  by  the  living  material  itself  but  by  the 
chemical  activities  of  substances  that  the  protoplasm  has  formed  by  its 
own  "vital"  activity,  in  the  same  way  that  it  produces  the  ferments, 
acids,  and  other  substances  mentioned  above. 

The  chief  difference  seems  to  be  that  the  latter  are  discharged  from 
the  cell  after  secretion  to  be  used  in  other  places,  while  the  heat,  light, 
and  electricity  producing  granules  are  used  inside  the  cell  (light  granules 
or  photochondria  are  sometimes  used  outside). 

By  secretion  we  shall  always  mean  the  elaboration  of  material  in  the 
protoplasm  by  the  activity  of  the  latter.  The  ejectment  of  this  material 
by  the  cell  or  the  gland  of  which  it  is  a  part  will  be  termed  discharge. 
Mathews  has  called  the  secretory  process  hyalogenesis,  and  the  particles 
so  formed  hyalogens.  This  is  done  to  avoid  using  "secretion"  in  the 
sense  that  we  use  it,  as  he  believes  this  word  to  mean  what  we  mean  by 
"discharge." 

The  above  discussion  reduces  us  to  the  idea  that  the  cell  can  produce 
only  a  substance  or  material.  The  method  of  producing  these  substances 
is  practically  not  known  at  all,  and  the  directive  force  that  controls  the 
activities  of  the  cell  is  entirely  unknown  and  a  subject  for  crude  specula- 
tion. 

Technic.  — The  most  ordinary  methods  carried  out  with  the  greatest 
care  are  the  best  to  use  in  the  study  of  protoplasm.  Flemming's  fluid, 
paraffin  sectioning,  and  iron  haematoxylin  staining  will  probably  give  the 
truest  pictures.  Living  material,  as  the  bodies  of  undifferentiated  Pro- 
tozoa, etc.,  should  also  be  studied  under  a  very  moderate  pressure. 
These  latter  must  be  examined  under  an  immersion  lens  with  the  dia- 
phragm reduced  to  get  the  best  optical  results.  A  particularly  beautiful 
picture  of  protoplasmic  structure  may  be  obtained  by  cutting  well-fixed 
material  in  celloidin,  and  either  staining  in  bulk  or  after  sectioning. 
The  lack  of  shrinkage  in  such  sections,  and  the  fact  that  one  sees  a  deeper 
layer  than  in  the  very  thin  paraffin  sections,  secures  a  picture  that  should 
be  studied  in  connection  with  the  paraffin  preparations.  One  should 
also  examine  the  tissue  alive  in  salt  solution.  Pressure  is,  then,  some- 
times necessary  to  get  a  thin  enough  layer  to  work  with. 

SOME   LITERATURE   ON   PROTOPLASM 

BUTSCHLI,   O.,    1892.     Untersuchungen  iiber  mikroskopische  Schaume  und  das  Proto- 

plasma.     Leipzig,  Wilhelm  Engelmann. 
WILSON,  E.  B.,  1899.     "The  Structure  of  Protoplasm,"  Journal  of  Morphology,  XV. 


CHAPTER  II 
THE    CELL 

PROTOPLASM,  as  it  is  ordinarily  encountered  in  living  things,  is  always 
organized  as  certain  structural  units  in  which  it  shows  some  characteristic 
differentiation.  The  cell  is  such  a  working  unit  of  protoplasm  (see  Fig. 
i).  These  living  protoplasmic  units  are  the  structural  units  of  which  all 
organisms  are  formed.  Living  units  of  an  extremely  low  type  have  been 
described,  which  consist  of  simple  undifferentiated  protoplasm.  These 
forms  are  not  very  frequently  encountered,  and  it  is  probable  that  the 

lack  of  differentiation 
is  due  to  the  ineffi- 
ciency of  the  methods 
employed  in  their 
study. 

The     simplest     of 

FIG.  4.  —  Tetramitus   chilomonas,  a  unicellular  animal  which       fUpcp    rpllc    nrp>    r^nrp 

.  .  .  .    .    .  ,.       „  .  ...  .  LllCbC      LLllb     die     IcUlC- 

shows  the  nuclear  material  in  a  distributed  condition ;  chr.,  .  L 

chromatin  masses.     From  CALKINS.  SCnted     in     SOme     low 

forms   of   plants    and 

animals  (Fig.  4).  In  these  low  forms  a  certain  differentiation  of  the 
protoplasm  is  detected  by  staining  processes.  Scattered  throughout 
the  general  protoplasm  of  such  a  unit  are  numerous  rounded  granules 
which  stain  deeply,  while  the  remaining  substance  does  not  stain.  The 
granules,  because  of  their  affinity  for  stain,  have  been  called  chromatin. 
The  non-staining  part  of  such  a  structural  unit  is  spoken  of  as  cytoplasm. 
A  structural  unit  or  cell  of  this  character,  which  has  chromatin  diffused 
or  scattered  throughout  its  cytoplasm,  is  called  by  some  a  pseudocyst 
or  false  cell.  Figure  4  shows  such  an  animal  in  which  the  chromatin 
appears  distributed  throughout  the  cell. 

Except  for  these  low  forms  of  life,  the  unit  of  structure  is  always  a 
more  highly  specialized  mass  of  protoplasm.  Part  of  the  protoplasm  of 
these  higher  cells  has  been  differentiated  to  form  a  specific  mass  that 
always  is  found  within  the  less  highly  specialized  protoplasm.  This 
is  known  as  the  nucleus,  and  the  protoplasm  within  which  it  lies  receives 
the  term  cytoplasm.  The  nucleus  itself  is  clearly  defined  as  a  round 
to  irregularly  shaped  body,  more  dense  than  the  cytoplasm.  It  is  by  no 
means  homogeneous.  The  modified  protoplasm  or  karyoplasm  of  which 

6 


THE    CELL 


•cy. 


it  is  composed  has  undergone  further  differentiation,  as  may  be  seen  in 
Figure  i. 

The  nucleus  is  clearly  defined  from  the  cytoplasm  by  a  film  of  spe- 
cialized karyoplasm  which  has  generally  been  looked  upon  as  having  a 
membranous  texture  and  has,  therefore,  received  the  name  nuclear  mem- 
brane. The  nature  of  this  membrane  has  not  yet  been  satisfactorily 
determined.  The  nuclear  membrane  incloses  a  transparent,  refractive 
fluid,  the  hyaloplasm. 

Supported  by  the  hyaloplasm  is  a  network  of  non-staining  refractive 
threads  which  form  a  scaffolding  upon  which  other  karyoplasmic  struc- 
tures are  distrib- 
uted.  These 
threads  are  com- 
posed of  the  linin 
of  the  nucleus. 

Tangled  with- 
in  the  linin 
meshes  are  usu- 
ally to  be  seen 
from  one  to 
many  rather 
large,  rounded 
bodies  which 
stain  more  read- 
ily than  the  linin 
and  are  relatively 
dense  and  highly 
refractive.  These 

are    the    nucleoli    FIG.  5.  —  Dorsal  nerve  cell  from  cord  of  Pterophryne  histrio.    ca.,  blood 
Or     filasmOSOmeS         capillaries  containing  blood  corpuscles;  n.,  nucleus  of  nerve  cell;  ca.n., 
nuclei  of  capillary  wall;  cy.,  cytoplasm.     X  400. 

Their    form   va- 
ries.    In  some  instances  they  are  rod-shaped,  in  othe'rs  they  become 
angular  lumps.    Their  texture  is  homogeneous.    They  frequently  in- 
close vacuoles  (Fig.  5). 

The  most  essential  parts  of  the  karyoplasm  are  the  deeply  staining 
granules  which  are  supported  upon  the  linin  network.  These  are  quite 
probably  the  homologues  of  the  deeply  staining  granules  of  pseudocysts, 
and  they  have  suggested  the  name  for  the  latter,  they  having  been  called 
chromatin.  The  chromatin  in  a  cell  that  is  not  dividing  is  distributed 
through  the  nucleus  in  a  number  of  particles  of  irregular  shape,  as  in 
Figure  i.  These  chromatin  grains  are  usually  many  and  are  clearly 
defined;  but  they  may  be  either  few  and  relatively  large  or  so  extremely 
minute  that  they  are  individually  invisible  and  give  a  cloud- like  appear- 


8 


HISTOLOGY 


ance  to  the  nucleus.  Sometimes  certain  portions  of  the  chromatin  are 
in  the  form  of  bodies  that  greatly  resemble  nucleoli  (Fig.  6).  Although 
such  chromatin  bodies  and  nucleoli  both  stain  deeply,  the  shade  and 
quality  of  stain  show  them  to  be  different.  The  nucleoli  or  plasmosomes 


ch  nit, 


iden. 


FlG.  6.  —  Motor  nerve  cell  from  electric  lobe  of  brain  of  Tetronarce.     imp.c.,  implantation  cone 
and  beginning  of  neurite;  den.,  dendrites;  ch.nu.,  chromatin  knot,     x  1500. 

stain  less  intensely  than  the  chromatin  masses.  These  chromatin  masses 
are  called  karyosomes  or  chromatin  knots  (see  Fig.  6).  Being  composed 
of  a  number  of  individual  chromatin  grains,  they  are  rough  and  irregular, 
while  the  plasmosomes  are  smooth  in  outline.  The  nucleus  sometimes 
contains  another  structure,  the  centrosome,  which,  as  it  is  usually  found 
lying  outside  the  nucleus  in  the  cytoplasm, 
will  be  described  in  connection  with  the 
cytoplasm. 

The  cytoplasm  may  be  looked  upon  as 
the  least  differentiated  part  of  the  cell  of 
higher  organisms.  The  general  study  of 
protoplasm  has  mostly,  if  not  always,  been 
based  upon  the  cytoplasm.  Hence  what 
was  said  in  Chapter  I  of  the  texture  of 
protoplasm  will  apply  to  cytoplasm.  It 
usually  presents  an  alveolar  appearance, 
der.  sec  secretion  substance;  fi.,  The  alveoli  may  be  so  small  that  the  cyto- 

secretion fibrils.  After  MATHEWS.          .  .         ,        .  __     ' 

plasm  appears  not  to  be  alveolar.  Numer- 
ous minute  granules  always  form  part  of  the  cytoplasm.  They  are  the 
microsomes  (see  Fig.  i).  Fibrillae  are  frequently  present.  They  are  to  be 
considered  as  differentiated  cytoplasmic  structures  which  have  to  do 
with  certain  activities  of  the  cell.  They  seem  to  be  closely  associated 
with  certain  forms  of  cytoplasmic  activity,  with  secretion  and  excretion, 


sec.  fi.-- 


FIG.  7. — Pancreas  cell  from  salaman- 


THE    CELL 


and  nuclear  division  (Fig. 
7,  and  see  Figs.  34  and 
137).  The  cytoplasm  is  the 
vegetative  part  of  the  cell  .  -  ,, 
and  is  frequently  charged 
with  'foodstuffs,  such  as 
yolk  granules  and  starch, 
and  with  secretion  and 
excretion  products  (Figs. 
8  and  9).  Plastids  and 
chloroplasts  are  found  most 
frequently  in  plant  cells 
(Fig.  10).  Vacuoles  con- 
taining fluids  are  frequently 
found  in  animal  cells  and 
are  always  present  in  a 
mature  plant  cell.  Within 
the  vacuoles,  crystals  and 

Other    Solids   are    Sometimes 

.  .  .       ' 

Stored  (see   rig.  9)- 

A  Very  important  StrUC- 
.  ..  , 

ture    is    sometimes    found 


ncl. 


G-  8"  —  Gland  cell  from  leech,  Pisicola.  sec.,  secreted  ma- 
terials  in  various  stages  of  elaboration  ;  ncl.,  nucleolus; 
««•,  nucleus  ;cy/.c/t.,cytoplasmic  channels  containing  and 
delivering  the  secretion  granules  to  the  large  distal  vacu- 
ole;  di.tu.,  discharging  tubes  of  this  and  two  other  cells. 


in  the  cytoplasm  as  a  permanent  feature;  sometimes  it  is  but  a  tem- 
porary structure,  arising  de  novo,  and  in  other  cases  it  may  be  absent. 


FIG.  9.  — Section  through  parts  of  two  regions  of  lobster's  nephridium.     ex.p.,  excretory 
products;  ex.-v.,  excretory  vacuoles.     X  425. 


10 


HISTOLOGY 


FIG.  10.  —  Cell  from  root  cap  of  calla  lily,     pi.,  plastids.     X  700. 


This  is  the  centrosome  (Fig.  n).  It  consists  of  a  number  of  radiating 
fibers,  which  may  be  absent  when  the  centrosome  is  quiet;  they  form 
the  aster.  These  fibers  converge  about  a  dense  area  in  the  cytoplasm, 

the  attraction  sphere 
(Fig.  n).  Within  the 
attraction  sphere  there 
are  present  one  or 
more,  usually  two, 
minute  bodies  called 
the  centrioles. 

In  a  living  cell, 
the  cytoplasm  and 
the  karyoplasm  or 
nucleus  are  vitally 
related.  The  nucleus 
is  influenced  by  the 
cytoplasm.  Different 
parts  of  the  cyto- 
plasm, indeed,  seem 
to  affect  differently 
the  dividing  nucleus  in  such  a  manner  as  to  determine  the  orientation 
or  position  of  the  nucleus,  as  has  been  observed  by  Lillie  in  the  dividing 
cells  of  Unio  eggs.  This  gives  rise  to  a  definite  axis  in  the  cell,  and  it 
is  then  said  to  have  polarity. 

Axes  of  cells  are  also  deter-     tf$$^$$y^  <%n.  sp. 

mined  by  secretory  struc- 
tures. In  epithelial  cells  of 
excretory  or  secretory  func- 
tions, fibrils  and  excretion 
particles  appear.  These  and 
the  nucleus  in  such  cells  do 
not  lie  promiscuously  within 
the  cell,  but  they  have  a 
definite  arrangement.  The 
fibrils  run  from  the  free  sur- 
face toward  the  base  of  the 
cell,  and  in  many  forms  con- 
verge to  a  common  point. 
The  elaborated  products  ap- 
pear at  various  levels  of  the 
cell  and  move  toward  the  free  surface  either  bodily  as  granules,  or  in 
invisible  solutions.  Thus  a  longitudinal  axis  is  established  for  the  cell. 
With  reference  to  this  axis,  cells  may  be  radially  or  bilaterally  sym- 


•centr. 


FIG.  ii.  —  Spermatogonium  of  salamander,  Spelerpes 
ruber,  containing  a  centrosome.  centr.,  centrioles;  cen. 
sp.,  centrosphere  (composed  of  an  inner  and  an  outer 
zone). 


THE    CELL  II 

metrical.     Haidenhain  speaks  of  cells  with  such  polarity  as  being  dorso- 
•ventrally  symmetrical. 

The  nucleus  cannot  live  outside  of  the  cytoplasm.  In  general  the 
nucleus  is  found  lying  in  the  cell  where  it  has  the  best  opportunity  for 
the  most  extensive  and  intimate  contact  between  its  surface  and  the 
main  cytoplasmic  mass.  It  usually  lies  in  a  central  position  in  the  cyto- 
plasm, but  it  may  lie  at  the  extreme  periphery.  In  fact  it  sometimes  lies 
so  far  out  that  it  occupies  a  position  outside  of  the  general  outline  of  the 
cell,  and  is  covered  by  a  mere  film  of  cytoplasm.  In  turn  the  cytoplasm 
cannot  live  for  long  or  reproduce  itself  without  the  nucleus. 

In  plants  the  cell  is  usually  inclosed  within  a  cellulose  cell-wall.  In 
animal  cells  there  is,  as  a  rule,  but  an  indefinite  cell-membrane;  most  of 
the  figures  of  animal  cells  will  show  this.  In  many  tissues  the  cell-walls 
or  cell-membranes  are  not  present.  This  results  in  a  blending  of  the 
cytoplasm  so  that  the  number  of  individual  cells  can  only  be  determined 
by  the  nuclei;  the  cell  boundaries  in  such  tissues  cannot  be  determined. 
Such  a  mass  of  cytoplasm,  with  frequent  nuclei,  is  called  a  syncytium. 

One  of  the  features  of  cell  organization  is  size.  With  very  rare  excep- 
tions the  cell  is  a  very  small  body  only  a  few  thousandths  of  a  millimeter 
in  diameter,  and  while  (relatively)  some  cells  may  be  twice  or  ten  times 
as  large  as  some  others,  yet  they  nearly  always  remain  microscopic 
bodies.  In  the  few  cases  where  they  are  macroscopic  objects,  as  the 
hen's  ovum  and  certain  low  plants,  we  find  that  this  unusual  size  is  due, 
not  to  a  larger  mass  of  protoplasm, 
but  to  non-living  contents  or  to 
internal  vacuoles  or  spaces.  The 
structure  of  protoplasm  evidently 

prohibits   its   working    in    more   tn- : 

than  a  certain  sized  mass,  nor 
is  this  an  arbitrary  rule.  The 
interchange  of  material  between 

cell  mass  and  exterior,  which  is       •.. 
constantly  and  necessarily  taking  «• 

place  in  living  protoplasm,  would 
alone  give  us  good  grounds  for 
this  conception,  when  we  re- 
member the  fact  that  the  surface, 

,,  i          ,  .   ,  ,    ...  FIG.   12.  —  .>erve   cell   from    sieilate  ganglion   of 

through    Which    nutritive,    waste,        squid>  Loligo  FealiL   /r.,  trophospongia  or  intra- 
and    Other    materials    must    pass,        cellular   blood    channels;    conn.t.fi.,  connective 

increases   as   the   square,   while     ^J^1*  ""  ""•  ^''^  * 
the  content  or  mass,  which  must 

be  supplied,  increases  as  the  cube  of  the  dimension.    This  idea  is  much 
strengthened  when  we  remember  that  most  of  the  very  largest  solH 


12  HISTOLOGY 

cells  known  have  increased  their  available  surface  by  invaginations, 
which  form  channels  that  carry  lymph,  blood,  and  waste  matter.  They 
thus  avoid  the  penalty  of  their  size  at  the  expense  of  extreme  specializa- 
tion. Figure  5  shows  such  a  cell  with  a  vascular  circulation.  Other 
channels  are  found  inside  some  cells  which  are  used  for  internal  trans- 
portation or  for  the  ultimate  gathering  of  fluid  and  solid  secretions  and 
their  removal  to  gland  lumina.  These  have  been  called  trophospongia 
by  Holmgren,  their  discoverer,  and  they  may  be  seen  in  Figure  12  as 
well  as  other  figures. 

The  shape  of  the  cell  varies  more  greatly  than  its  size.  Cells,  where 
free  from  distorting  lateral  pressure,  are  as  nearly  spherical  as  possible. 
Their  position  and  use  often  cause  them  to  assume  very  extraordinary 
shapes,  such  as  flat  disks,  long  rods,  and  extreme  branching  forms. 

Technic.  — The  same  remarks  that  followed  the  part  on  protoplasm 
will  apply  here.  To  get  an  accurate  conception  of  the  organization  of  any 
kind  of  a  cell,  it  is  necessary  to  not  only  prepare  this  cell  by  a  number 
of  the  best  methods,  but  to  also  make  many  preparations  by  the  same 
method.  These  latter  preparations  will  differ  much  among  themselves 
as  to  the  form  of  the  different  organs  and  the  staining  powers  of  the 
different  cell-substances. 

Flemming's  strong  fixing  fluid  is  perhaps  the  best  fixative  known  in  the 
majority  of  cases.  It  requires  the  most  care  and  skill  in  its  use,  but  gives 
the  truest  pictures  of  the  materials  prepared  by  its  agency.  Zenker's 
fluid,  chrom-aceto-formol,  and  picro-acetic  are  examples  of  some  of  the 
best  of  the  other  fluids,  and  they  should  all  be  tried.  . 

LITERATURE 

The  subject  is  so  extensive  and  the  literature  consequently  so  large  that  the  student  is 
referred  to  the  general  text-books.  Wilson,  "The  Cell,"  Schneider,  " Lehrbuch  der 
Histologie,"  and  Hertwig's  "Cell"  will  give  extensive  reading. 


CHAPTER  III 
MULTICELLULAR  ORGANIZATION:   PHYLOGENETIC 

ACCORDING  to  the  doctrine  of  descent,  which  at  present  receives  wide 
recognition,  all  animals  have  evolved  from  some  simpler  forms.  It  is 
held  that  every  multicellular  animal  or  plant  has  developed  through  an 
infinite  series  of  stages  from  a  unicellular  form.  The  geological  forma- 
tions bear  broken  records  of  such  an  advance  in  structure.  This  past 
history  of  a  race  is  called  its  phylogeny.  Necessarily  it  is  a  history  of 
which  we  have  no  human  records,  since  man  did  not  exist  during  the 
longer  early  periods  and  was  not  a  scientific  investigator  until  a  com- 
paratively recent  time.  Our  only  actual  evidence  lies  in  comparative 
study  of  the  fossil  remains  of  such  creatures  as  happened  to  be  preserved 
in  the  rocks.  Of  these  we  have  only  the  hard  parts,  as  bone  and  shell 
in  most  cases,  and  but  few  actual  histological  structures  have  been  pre- 
served, as  in  the  case  of  some  selachian  muscle  and  some  crustacean 
integument. 

These  fossil  remains  show  that  in  earliest  times  only  the  simplest 
forms  existed,  and  that,  as  ages  passed,  larger  as  well  as  more  complex 
forms  were  added.  Some  races  were  entirely  lost  during  these  changes. 
The  geological  record  also  shows  that  the  simplest  and  intermediate 
groups  continued  to  exist  with  little  change  to  the  present  time,  except 
some  that  have  been  lost  entirely.  Therefore,  we  may  assume  that  the 
present  large  series  of  animals,  known  as  the  taxonomic  series,  represents, 
to  a  certain  degree,  the  long  extinct  phylogenetic  series  whose  broken 
record  we  find  in  the  rocks. 

The  taxonomic  series  suggests  that  higher  efficiency  in  organization 
has  been  effected  along  two  lines :  First,  by  an  increase  in  the  mass  of 
the  individual;  and  secondly,  by  a  differentiation  of  the  component  cells 
of  an  individual.  How  these  modifications  were  accomplished  can,  to  a 
certain  degree,  be  .understood  by  a  study  of  certain  living  forms. 

We  have  seen  in  the  preceding  chapter  that,  though  the  specialization 
in  cellular  organization  be  varied,  the  maximum  size  of  cells  is  soon 
reached.  It  is  only  exceptionally  that  unicellular  creatures  attain 
macroscopic  dimensions.  Increase  of  bulk  is,  therefore,  rarely  effected 
by  the  growth  of  a  single  cell  and  is  usually  accomplished  by  the 

13 


14  HISTOLOGY 

grouping  of  cells  to  form  a  colony.  In  such  colonization  of  cells  we  have 
represented  the  first  step  toward  multicellular  organization.  The  rela- 
tion of  the  cells  to  each  other  in  these  multicellular  organisms  varies  with 
the  degree  of  advancement  the  colony  has  attained.  This  affords  a 
basis  for  dividing  multicellular  organisms  into  three  orders  of  coloniza- 
tion. The  cells  constituting  any  one  of  these  kinds  of  colonies  should 
usually  be  considered  as  descendants  of  a  single  cell  or  otherwise  closely 
related. 

In  a  colony  of  the  first  order  the  component  cells  have  no  intimate, 
vital  relation  with  one  another.  This  colonization  results  in  a  -mere 
increase  in  bulk,  by  which  the  cells  are  mutually  protected  against  the 
disturbing  forces  of  their  environment.  Each  cell  of  such  a  colony  is 
capable  of  living  alone,  if  detached  from  the  colony;  and  the  colony 
exists  with  an  indefinite  number  of  cells.  In  this  case  the  cell  and  not  the 
colony  must  be  considered  the  individual.  Such  colonies  are  found  rep- 
resented by  certain  Protophyta  and  Protozoa. 

In  the  second  order  of  cell-colonies  we  find  a  more  intimate  relation 
existing  between  the  members  of  the  cell  aggregation.  In  a  colony  of 
this  type  the  number  of  component  cells  is  always  constant.  The  death 
of  one  or  more  of  the  cells  must  result  in  their  replacement  by  new  cells. 
In  such  a  colony  one  cell  cannot  move  independently  of  the  others.  This 
is  a  comparatively  simple  mode  of  colonization.  No  cell  is  here  con- 
cerned in  a  peculiar  manner  with  the  life  of  the  colony,  each  cell  per- 
forming all  of  the  vital  functions.  Algae  such  as  Pandorina,  Eudorina, 
and  Gonium  present  this  type  of  colonization. 

The  third  and  highest  order  of  colonization  or  cell  aggregation  presents 
groups  of  cells  vitally  related  to  one  another  and  in  which  the  cells  are  not 
all  alike.  A  certain  amount  of  differentiation  has  taken  place,  with  the 
result  that  certain  cells  with  changed  character  are  set  apart  to  perform 
definite  functions.  This  is  the  type  of  cell-colony  met  with  in  all  the 
Metazoa.  Phylogenetically  this  order  of  cell-colony  probably  would 
fall  under  two  divisions.  One  division  would  include  the  colonies  in 
which  the  differentiated  cells  remained  independent  of  their  fellows 
which  performed  the  same  function.  The  reproductive  cells,  for  exam- 
ple, were  not  confined  to  a  particular  region  of  the  colony,  but  were  scat- 
tered independently  throughout  the  cell  aggregate.  An  example  of  this 
simple  multicellular  organism  of  the  third  order  may  be  seen  in  Volvox 
globator.  In  the  second  division  of  this  third  order  of  colonization  we 
meet  with  a  higher  grade  of  organization.  In  a  cell  aggregate  of  this 
type  the  cells  differentiated  to  perform  a  particular  function  are  assembled 
in  a  particular  part  of  the  body  of  the  animal.  The  segregation  of  cells 
similarly  modified  has  given  rise  to  what  is  known  in  histology  as  a  tissue. 
A  tissue,  then,  is  an  aggregation  of  cells  that  have  been  specialized  to 


MULT1CELLULAR    ORGANIZATION:   PHYLOGENETIC 


perform  or  help  perform  a  definite  function.  As  an  example  of  an  animal 
belonging  to  this  second  division  of  the  third  order  of  multicellular 
organization,  we  may  cite  Hydra. 

Multicellular  organization  in  the  narrower  sense  of  the  term  ends  here ; 
but  this  does  not  cover  the  scope  of  multicellular  organization.  Just  as 
cells  are  assembled  to  form  a  colony  in  which  certain  of  them  are  differen- 
tiated and  segregated  to  form  tissues,  so  tissues  have  been  differentiated 
and  segregated  to  perform  definite  functions  or  sets  of  functions,  giving 
rise  to  organs.  An  organ,  then,  is  a  group  of  differentiated  tissues  per- 
forming some  particular  function. 

Multicellular  organisms,  therefore,  range  from  groups  composed  of 
an  indefinite  number  of  cells  scarcely,  if  at  all,  related  to  each  other,  to 
individuals  composed  of  closely  interrelated  and  mutually  dependent 
organs.  It  is  also  held  that  the  phylogenetic  series  was  equally  as  exten- 
sive. This  is  rendered  all  the  more  likely  by  the  evidence  gathered  from 
the  study  of  the  ontogenetic  series. 

An  Example  of  a  Colony  of  the  First  Order:  Carchesium.  —  The  indi- 
vidual cells  in  a  colony  of  Carchesium  are  attached  to  a  branched  system 
of  stalks  which  they  themselves  have  elabo- 
rated. There  is,  in  this  system  of  stalks, 
a  set  of  radiating  branches  that  arise  from 
a  common  point  of  attachment.  From  these 
radial  branches  short  lateral  branches  are 
given  off  at  more  regular  intervals.  At  the 
end  of  each  radial  and  each  lateral  branch 
a  single  cell  is  borne  (Fig.  13).  Each  ani- 
mal is  a  bell-shaped  cell  measuring,  when 
extended,  about  fifty  microns  in  diameter 
and  seventy-five  microns  in  length.  The 
cytoplasm  is  differentiated  into  an  ectoplasm 
and  an  endo plasm.  Within  the  endoplasm 
there  is  a  constant  cyclosis.  This  endoplasm 
contains  a  sausage-shaped  macronucleus 
and  a  single  rounded  micronucleus.  These 
nuclei  are  not  carried  with  the  cyclosis,  but 
have  a  stationary  position  in  the  cell.  Open- 
ing out  from  the  ectoplasm  is  a  single  con- 
tractile vacuole.  At  the  distal  end  there  is 
a  ciliated  zone  which  makes  about  one  and  a  half  spirals  as  it  winds 
about  the  body  to  enter  the  "gullet."  At  the  base  of  the  gullet  the 
food  passes  through  the  mouth  into  the  endoplasm  to  form  a  food 
vacuole.  The  stalk  of  each  animal  is  provided  with  a  contractile  fiber 
(Fig.  13,  /.)• 


f! 


FIG.  13.  —  A  colony  of  individuals 
of  Carchesium  attached  by  a  com- 
mon branching  stalk ;  /,  contract- 
ile stalk  of  one  individual.  (REMY 
after  PERKIER.) 


i6 


HISTOLOGY 


Each  animal  lives  at  the  end  of  its  stalk  independently  of  the  others 
/  of  the  body.     It  secures 

its  food  by  means  of  its 
own  ciliary  movements 
and  carries  on  for  itself 
all  metabolic  and  repro- 
ductive functions  inde- 
pendently of  the  colony 
as  a  whole. 

An  Example  of  a  Col- 
ony of  the  Second  Order : 
Gonium  pectorale.  —  A 
specimen  of  Gonium 
pectorale  is  always  com- 
posed of  sixteen  oval 
cells  attached  laterally 
to  one  another  in  such 

FIG.  14.  —  Gonium  pectorale,  showing  the  individuals.  (From    a   manner   as   to    form   a 

square  colony  with 

twelve  cells  on  the  margin  of  the  square  and  four  cells  inclosed  by  the 
lateral  ones.  The 
entire  colony  is 
surrounded  by  a 
gelatinous  sheath. 
Each  cell  is  oval 
and  inclosed  within 
a  cellulose  wall. 
The  cytoplasm  con- 
tains a  cup-shaped 
green  chloroplast 
and  a  centrally 
placed  nucleus. 
Near  the  open  mar- 
gin of  the  chloro- 
plast there  is  a 
bright  red  stigma. 
In  the  cytoplasm 
opposite  the  open- 
ing of  the  chloro- 
plast are  two 
contractile  vacu- 
oles.  From  this 


FlGg  JS'  —  Volvox  globator,  a  colonial  organism  of  the  third  order. 
(From  WILSON,  after  J.  H.  EMERTON.) 


same  region  of  the  cell  two  slender  flagella  are  given  off  (Fig.  14). 


MULTICELLULAR    ORGANIZATION:  PHYLOGENETIC 


It  is  to  be  noted  in  this  connection  that  here  no  cell  has  the  power  to 
move  independently  of  its  fellows. 

An  Example  of  Colonies  of  the  Third  Order;  First  Division:  Volvox 
globator.  — This  creature  is  spherical,  with  all  the  cells  confined  to  the 
surface  of  the  sphere  so  that  the  sphere  has  a  cellular  wall  and  a  cavity 
bearing  no  cells.  A  Volvox  colony  of  ordinary  size  measures  three  hun- 
dred microns  in  diameter.  There  is  no  definite  number  of  cells  in  a 
colony,  so  that  the  size  varies  greatly;  a  colony  may  measure  over  seven 
hundred  microns  or  even  a  thousand  microns  in  diameter.  Each  cell  has 
all  the  parts  described  as  belonging  to  a  cell  of  Gonium  pectorale.  The 
cells  are  united  to  each  other  by  protoplasmic  strands  radiating  from  each 
cell  (Fig.  15).  The  colony  revolves  on  a  definite  axis,  and  moves  toward 
one  of  its  poles.  The  stigmata,  which  probably  have  to  do  with  the  recep- 
tions of  light  impressions,  are,  therefore,  always  located  in  each  cell  to 
face  in  the  direction  of  travel  as 
much  as  their  position  in  the  cell- 
colony  will  permit.  It  is  of  chief 
importance  here  to  note  that 
certain  cells  are  modified  to  per- 
form sexual  functions.  Any  cell 
of  the  mass  may  become  a  female 
cell  or  give  rise  to  a  group  of 
male  cells.  The  female  cell  is  a 
large  spherical  cell  free  from 
flagella  and  provided  with  a 
large  nucleus  and  a  highly 
granular  cytoplasm.  This  cell 
is  called  an  ovum  (see  Fig.  15). 
Our  figure  shows  the  primitive 
germ  cells.  By  repeated  divi- 
sion, certain  of  these  germ  cells 
become  separate  clusters  of 
small,  colorless,  spindle-shaped 
cells  with  two  flagella.  These 
are  the  male  cells  or  sperm- 
cells.  It  is  to  be  seen  here 
that  the  ova  are  distributed 
promiscuously  throughout  the 
vegetative  cells.  There  is  ap- 
parently a  tendency  to  as- 
semble cells  differentiated  in 
a  particular  manner  seen  in  the  groups  of  sperm-cells;  but  this  is  only 
due  to  their  mode  of  origin.  The  clusters  of  sperms  arising  from 


end. 


FIG.  16.  —  Part  of  the  body  of  a  sponge,  Gran- 
tia:  mes.,  mesoderm;  ect.,  ectoderm;  end., 
endoderm;  spi.,  spicules. 


1 8  HISTOLOGY 

widely  separated  cells  are  not  assembled  to  form  a  definite  region  in 
the  colony. 

Second  Division:  Grantia  sp. — In  Grantia  and  other  sponges  we  meet 
with  a  triploblastic  colony ;  by  this  we  mean  a  cell  aggregate  composed 
of  three  layers  of  cells.  The  outer  layer  of  cells  forms  a  thin  layer  of 
nucleated  cytoplasm  (Fig.  16,  ect.).  The  inner  layer  is  composed  chiefly 
of  oval  cells  supplied  at  their  free  ends  with  a  bell-shaped  collar  and  a 
slender  flagellum  (Fig.  16,  end.}.  Between  them  lies  a  layer  of  greatly 
branched,  anastomosing  cells.  These  cells  bear  rounded  nuclei  (Fig.  16, 
mes.}.  We  observe  here  that  the  cells  modified  to  perform  the  function 
of  protection  are  assigned  to  a  particular  region  of  the  cell-colony  and 
that  they  form  a  continuous  outer  layer,  the  ectoderm  (ect.).  Likewise 
the  cells  performing  the  function  of  alimentation  are  assembled  and 
relegated  to  particular  regions  to  form  part  of  the  endoderm  (end.). 
Finally,  between  these  two  tissues  we  note  the  third  tissue  made  up  of 
cells  looking  chiefly  after  the  mechanical  support  of  the  colony  and  unit- 
ing to  form  a  loose  tissue  —  the  mesoderm  (mes.). 

LITERATURE 

Read  general  articles  and  parts  of  such  books  as  "The  Foundations  of  Zoology,"  by 
W.  K.  Brooks. 


CHAPTER   IV 
MULTICELLULAR    ORGANIZATION:    ONTOGENETIC 

THE  developmental  history  of  a  multicellular  animal  from  the  egg  to 
full  maturity  is  known  as  its  ontogeny.  The  soma  or  body  of  such  an 
animal  is  thus  an  aggregate  of  cells  descended  by  successive  divisions 
from  a  single  cell,  the  fertilized  ovum  or  oo sperm.  These  cells  form 
connected  masses,  resulting  in  one  or  more  bodies  or  individuals,  and 
in  such  an  individual  it  will  be  noticed  that  two  things  have  occurred. 

First.  The  cells,  during  their  successive  generations,  have  grown 
to  be  of  several  different  kinds,  each  of  which  is  adapted  to  perform 
some  particular  function  that  the  individual  may  be  called  to  maintain, 
and, 

Second.  These  different  kinds  of  cells  have  been  grouped  apart  or 
together  to  form  regions  and  relations  with  each  other  that  will  best 
permit  them  to  perform  their  peculiar  functions.  Such  regions  or  asso- 
ciations of  cells,  together  with  their  products,  are  called  tissues;  or, 
where  certain  tissues  are  very  especially  arranged  apart  from  other  tis- 
sues, the  tissue  aggregate  is  known  as  an  organ. 

These  two  conditions  are  known  as  differentiation  and  organization 
respectively,  and  the  ability  of  the  creature  to  maintain  its  life  and  place 
in  the  world  depends  upon  the  degree  of  efficiency  which  differentiation 
and  organization  have  attained  in  relation  to  the  conditions  under  which 
the  creature  exists. 

One  of  the  most  important  differentiations  occurs  early  in  the  life 
history  and  represents  the  separation  of  all  the  cells  that  are  to  undergo 
further  differentiation  from  one  or  more  of  them  that  do  not  differentiate 
fundamentally  bjit  remain  as  a  store  of  the  original  material,  to  be  used 
later  in  building  up  other  organisms  of  the  same  kind  (see  Chapter 
XXI).  These  latter  are  called  the  reproductive  cells  (Fig.  17).  They 
undergo  a  very  special  differentiation  of  their  own  at  their  time  of 
maturity.  They  may  be  many  or  few,  and  some  of  them  sometimes 
appear  to  not  only  retain  their  original  reproductive  powers,  but  to  also 
perform,  in  a  degree,  some  of  the  body  functions.  These  may  be  con- 
sidered, therefore,  under  such  circumstances,  as  somatic  cells.  In  this 
case  we  have  a  slight  differentiation  as  compared  with  the  higher  forms 

19 


20 


HISTOLOGY 


in  which  the  somatic  cells  are  so  strongly  and  so  early  differentiated 
from  the  reproductive  cells  that  they  cannot  retain  their  power  of 
reproduction  and  perform  somatic  duties  at  the  same  time. 

In  the  first  examples  the  groups  of  somatic  cells  are  usually  also 
slightly  differentiated  from  each  other,  while  in  the  latter  they  are  highly 
differentiated. 

This  differentiation  does  not  all  come  about  at  once.  The  dividing 
oosperm  may  transmit  to  its  descendants  all  of  its  qualities  and  original 
powers  and  structures  for  a  considerable  number  of  cell  generations 
before  any  one  of  these  descendants  begins  to  differentiate.  Or  the  dif- 


FiG.  17. — A,  second  cleavage  division  of  the  oosperm  of  Ascaris,  showing  the  first  differentia- 
tion by  loss  of  chromatin  in  the  somatic  cell.  B,  resulting  four  cells,  showing  the  lost  chro- 
matin,  ch.,  and  the  smaller  resulting  nuclei  in  the  daughter  somatic  cells.  (From  WILSON 
after  BOVERI.) 

ferentiation  may  begin  by  changes  in  one  or  the  other  of  the  two  cells 
produced  by  the  first  cleavage  division.  In  fact  the  frequent  greater 
size  of  one  of  these  first  two  daughter  cells  of  the  oosperm  shows  that 
there  were  differentiating  forces  in  the  oosperm  itself  Before  it  began  to 
divide,  and  we  are  thus  brought  to  see  that  the  beginnings  of  differen- 
tiation are  sometimes  preformed  in  the  oosperm.  This  can  be  seen  in 
a  number  of  ways  in  the  developing  eggs  of  several  kinds  of  animals  and 
may  be  discussed  under  a  few  principal  headings. 

The  simplest  and  most  fundamental  form  is  that  seen  in  the  organisms 
whose  oosperms  show  a  distinct  polarity  in  their  organization,  as  in  the 
frog  and  many  other  animals.  A  feature  of  this  polarity  is  the  collec- 
tion of  the  yolk  or  food  supply  of  the  ovum  at  its  lower  end  and  of  the 


MULTICELLULAR    ORGANIZATION:    ONTOGENETIC  21 

chief  fundaments  of  its  future  nervous,  muscular,  and  other  organization 
at  the  upper  pole. 

This  might  not  appear  to  be  so  primitive  a  distinction  as  the  early 
evidence  of  bilateral  symmetry  seen  or  inferred  in  the  oosperms  of  other 
animals  were  it  not  for  the  fact  that  it  may  be  compared  functionally 
with  the  early  taxonomic  associations  of  individual  cells  mentioned  in 
the  preceding  chapter  and  serving  here  as  a  suggestion  of  phylogenetic 
history.  In  these,  the  use  of  the  lower  surface  of  the  mass  for  the  acqui- 
sition of  food  may  be  compared  to  the  storing  of  food  in  the  lower  part  of 
the  body  of  the  dividing  ovum  and  to  the  subsequent  development  of 
this  surface  by  imagination  into  the  chief  digestive  organs  of  the  body. 
This  imagination  of  the  lower  body  surface  is  known  as  gastrulation. 
By  this  process  the  body  mass  of  the  young  embryo  becomes  arranged  in 
an  upper  and  a  lower  layer  which  are  called  respectively  the  ectoderm  and 


FIG.  1 8.  — Transverse  section  of  very  young  cat  embryo: 
showing  ectoderm,  mesoderm,  and  endoderm.     X  400. 

endoderm.  Where  the  animal  is  highly  differentiated  and  a  somewhat 
more  complete  early  differentiation  is  needed,  a  third  layer  is  formed 
between  these  two  and  is  called  the  mesoderm  (Fig.  18). 

In  such  an  early  differentiation  we  have  certain  groups  of  tissues 
represented  by  the  three  layers,  and  to  this  extent  the  layers  are  appar- 
ently homologous  in  many  groups  of  nearly  related  animals.  But  ho- 
mology  breaks  down  to  such  an  extent  when  details  are  examined  that  it 
changes  into  an  analogy  when  some  larger  groups  are  compared,  'and  we 
come  to  see  that  the  analogy  is  based  upon  differentiations  that  are  re- 
sponses to  particular  conditions  under  which  any  embryos,  or  the  embryos 
of  any  group,  must  develop,  rather  than  a  fixed  type  of  development  which 
is  similar  because  of  blood  relationship. 

Protoplasm  is  slow  to  change  its  methods,  however,  and  blood  rela- 
tionship, or  common  descent,  probably  does  in  a  large  degree  determine 
many  similarities  of  embryological  development.  The  moment,  how- 
ever, that  we  attempt  to  compare  the  wider  groups,  we  must  recognize 
the  possibility  that  even  a  large  number  of  similar  processes  may  proceed 


22 


HISTOLOGY 


from  the  methods  used  to  meet  similar  conditions,  and  not  from  phylo- 
genetic  relationships,  other  than  the  common  possession  of  a  protoplasm 

which  is  subject  to  the  same  laws. 

The  early  bilateral  symmetry  of  the 
young  eggs  of  many  animals  is  a  pre- 
formation  of  the  form  of  the  body  best 
adapted  to  the  needs  of  motion  (Fig. 
19).  Where  there  is  no  ancestral  his- 
tory of  motion,  or  where  this  history  is 
very  far  in  the  background,  the  animal 
develops  with  an  upper  and  a  lower 
pole  and  a  number  of  radially  symmet- 
rical sides.  The  number  of  these  sides 
varies  from  four  sides  in  some  me- 
FIG.  19.  — Segmenting  ovum  of  Loiigo  dusae  to  the  greater  numbers  seen  in  the 

Pealii  to  show  early  traces  of  bilateral          •,  •        i  ,-,       ,-,  -,-,      •    r    • , 

symmetry,    x  45-   (After  WATASE.)      echinoderms  or  the  theoretically  infinite 

number  in  the  sessile  sponges. 

When  the  animal  is  one  that  is  to  be  moving  in  any  part  of  its  life 
history,  the  development  tends  toward  the  bilateral  symmetry  found 
in  all  animals  that  move  forward.  The  bilateral  symmetry  may  be 
superimposed  upon  the  radial  as  is  seen  in  the  adults  of  some  echino- 
derms, where  radial  symmetry  extends  into  many  planes,  while  that  same 
radial  structure  is  preceded  by  a  bilateral  form  in  the  early  embryo. 

In  the  smaller  and  less  highly  organized  forms  the  tendency  is  to 
form  organs  as  soon  as  possible,  that  the  creature  may  begin  an  inde- 
pendent life,  securing  its  food  and  escaping  its  enemies  by  various  motor 
and  protective  devices.  These  organs  are  often  temporary,  and  replaced 
by  other  forms  in  the  adult.  The  further  details  of  the  development  of 
tissues  will  be  considered,  where  they  are  necessary  to  a  real  under- 
standing of  histological  structure,  in  connection  with  the  descriptions  of 
the  various  organs. 

LITERATURE 


Read  the  parts  of  Wilson,  "The  Cell,"  Schneider,  " Lehrbuch  der  Histologie,"  and 
other  general  works  that  cover  this  subject. 


CHAPTER   V 
MITOSIS 

THE  growth  of  all  tissues  depends  primarily  upon  the  increase  in  the 
number  of  cells  constituting  the  tissue.  New  cells  arise  only  from  par- 
ent cells  by  division  of  the  parent  cells.  Since  the  life  of  any  cell  depends 
upon  the  presence  of  a  nucleus,  this  process  of  division  involves  a  divid- 
ing of  the  nucleus.  In  most  cases  of  cell  division  the  dividing  nucleus 
elongates  at  right  angles  to  the  plane  of  cytoplasmic  division.  There 
are  two  types  of  nuclear  division.  The  one  is  comparatively  simple  in 
the  number  of  phases  which  it  presents;  the  other  involves  a  series  of 
complex  nuclear  changes.  The  first  is  known  as  amitosis  and  is  con- 
sidered in  another  chapter ;  the  second  has  been  termed  by  various  writers 
mitosis,  karyokinesis,  and  indirect  division. 

The  nucleus  of  a  given  species  always  undergoes  in  its  mitosis  a 
definite  series  of  structural  changes.  The  mitoses  for  various  species 
present  considerable  variation.  As  mitosis  has  to  do  primarily  and 
essentially  with  an  equal  division  of  the  chromatin  of  a  mother  nucleus 
between  two  daughter  nuclei,  there  is  encountered  less  variation  in  the 
structural  phases  assumed  by  the  chromatin  than  in  the  other  structures 
concerned  with  mitosis. 

The  series  of  chromatin  phases  and  their  order  of  sequence  is  dia- 
grammatically  represented  by  Figure  20,  A  to  7.  The  chromatin,  which, 
in  a  resting  nucleus,  is  more  or  less  generally  distributed  within  the 
nucleus  as  granules  of  chromatin  (Fig.  20,  A),  is  assembled  to  form  in 
most  cases  a  chromatin  thread  known  as  the  spireme  (Fig.  20,  B).  The 
nucleus  now  elongates,  and  in  the  higher  forms  the  nuclear  membrane 
at  this  stage  disappears.  The  spireme  thread  breaks  into  a  number  of 
fragments.  These  may  be  rounded  to  rod-shaped  bodies.  Each  chro- 
matin segment  is  called  a  chromosome.  In  our  diagram  we  have  shown 
at  C  six  chromosomes  derived  from  the  spireme.  //  is  highly  probable 
that  the  number  of  chromosomes  in  a  nucleus  is  constant  for  a  given  species. 
For  example,  with  the  breaking  of  the  spireme  of  a  somatic  nucleus  from 
man,  sixteen  chromosomes  will  arise;  the  nucleus  of  Ascaris  megalo- 
cephala  var.  bivalens  represents  four  chromosomes.  In  certain  mitoses 
the  number  of  chromatin  segments  may  vary  from  the  specific  number. 
It  seems  to  be  supported  by  evidence  that  when  such  variation  occurs, 


24 


HISTOLOGY 


certain  of  the  chromatin  rods  or  all  of  them  represent  one  or  more  chro- 
mosomes fused.  These  compound  chromosome  rods,  when  formed  of 
two  chromosomes,  are  said  to  be  bivalent;  where  more  than  two  chro- 
mosomes enter  into  their  formation,  they  are  said  to  be  plurivalent. 

The  chromosomes  formed  from  the  segmenting  of  the  spireme  as- 
semble in  a  plane,  through  which  the  cell  will  divide,  forming  what  is 
termed  the  equatorial  plate.  In  cases  where  the  chromosomes  are  rod- 
shaped  they  usually  become  bent  into  V-shaped  bodies.  The  apices 
of  the  bent  chromosomes  often  converge  so  that  the  equatorial  plate, 
when  seen  from  above  or  below,  presents  a  radiating  figure  1  (D).  In 
this  position  each  of  the  mother  chromosomes  divide  by  splitting  longi- 


D 


FIG.  20.  —  A-I.    Diagrams  of  chromatin  changes  during  the  division  of  a  cell. 

tudinally  into  two  daughter  chromosomes.  Two  groups  of  daughter 
chromosomes  are  thus  formed  (£).  One  group  is  assembled  above  the 
plane  of  cytoplasmic  division,  the  other  below  at  right  angles  to  it  (F). 
Thus  two  groups  of  an  equal  number  of  chromosomes  are  brought  to  lie 
opposite  each  other  with  the  plane  of  approaching  cell  division  lying 
between  them  (G).  The  chromosomes  of  each  group  blend  with  one 
another  to  form  a  spireme.  About  each  spireme  a  nuclear  membrane 
.  forms,  and  we  have  the  dispireme  phase  of  the  chromatin  (H}.  The 
spiremes  next  become  more  slender  and  finally  break  up  into  chromatin 
particles  to  be  distributed  throughout  the  nuclear  space  (/). 

As  a  result  of  this  series  of  chromatin  changes  there  are  two  groups  of 

1  This  is  frequently  spoken  of  as  the  monaster.  The  word  aster,  however,  is  given  to 
another  part  of  the  mitotic  figure  ;  so  for  clearness  of  terms  we  do  not  here  use  the  word 
aster  for  any  figure  formed  by  the  chromatin. 


MITOSIS 


chromatin  in  the  same  structural  condition  as  was  the  chromatin  of  the 
resting  mother  nucleus  (/).  From  the  granular  condition  of  the  chro- 
matin of  a  resting  nucleus  to  the  formation  of  the  equatorial  plate 
there  is  a  progressive  series  of  chromatin  changes  which  carries  the  chro- 
matin toward  the  plane  of  cell  cleavage  (A  to  D  inclusive).  The 
splitting  of  the  chromosomes  at  this  plane  is  an  intermediate  phase. 
Following  this  there  is  a  regressive  series  of  chromatin  phases  during 
which  the  divided  chromatin  travels  away  from  the  plane  of  cell  divi- 
sion (F-G-H-1).  The  progressive  phases  we  choose  to  call  the  pro- 
phases;  the  intermediate  the  metaphase,  and  the  regressive  series  the 
anaphases.  The  chromatin  during  all  these  phases  stains  deeply ;  for 
which  reason  the  figures  presented  by  the  chromatin  in  mitosis  have 
been  called  the  chromatic  figures  of  mitosis. 

In  all  mitoses  there  are  other  structures,  much  less  constant  in  ap- 
pearance in  various  species,  which  do  not  stain  readily.  These  are  called 
the  achromatic  structures  or  figures  of  mitosis.  They  furnish  the  path 
along  which  the  series  of  chromatin  changes  are  run,  and  perhaps  they 
are  the  dynamic  structures  that  control  and  direct  the  chromatin  move- 
ments. The  chief  or  most  common  of  

these  is  the  spindle  (see  following  numer- 
ous figures).  Ordinarily  the  outline  of 
this  structure  is  fusiform.  The  shape, 
however,  is  not  constant.  It  may  be 
cylindrical-  to  barrel-shaped.  In  most 
cases  the  spindle  is  striated,  showing 
that  it  is  composed  of  delicate  fibrils 
arranged  longitudinally  and  converging 
at  the  poles.  In  a  typical  spindle  there 
is  a  central  bundle  of  fibrils,  the  spindle 
fibrils,  extending  from  pole  to  pole,  and 
a  second  set  of  fibrils  which  extend 
from  chromosomes  to  poles.  These  sur- 
round the  central  fibrils  and  have  re- 
ceived the  name  of  mantle  fibrils  (Fig. 
21).  The  spindle  always  lies  at  right 
angles  to  the  plane  along  which  the  cell 
divides,  with  its  equator  in  this  plane, 
so  that  it  is  about  the  equator  of  the  spindle  that  the  equatorial  plate  of 
chromosomes  is  formed.  As  the  daughter  chromosomes  separate,  a 
plate  of  granules  is  formed,  in  many  spindles,  through  the  equator 
(see  Fig.  34). 

These  granules  appear,  one  on  each  fibril,  and  they  unite  to  form 
collectively  a  structure  known  as  the  cell-plate.  The  cell-plate  spreads 


FIG 


21.  — Mitotic  figure,  spermatago- 
nium  of  Podophyllum.  S^hows  the 
spindle  fibrils  reaching  from  pole  to 
pole  and  the  thicker  mantle  fibrils 
reaching  from  pole  to  ends  of  chromo- 
somes. 


26 


HISTOLOGY 


with  the  advance  of  mitosis  in  such  a  manner  that  it  may  take  part  in 
the  formation  of  the  transverse  cell- wall  or  cell- membrane. 

In  most  animals  and  in  some  plants 
the  spindle  of  mitosis  has  at  each  pole 
an  aster  with  its  cytoplasmic  rays, 
archoplasm  and  centriole.  A  spindle 
with  its  asters  is  termed  an  amphi- 
aster.  There  is  usually  one  spindle 
arising  from  a  single  nucleus.  In 
certain  plant  tissues  and  in  pathologi- 
cal tissues  very  complicated  spindles 
arise  with  more  than  two  poles.  In 
the  nurse  cells  of  magnolia,  for  ex- 
ample, tripolar  spindles  are  met  with 

FIG.  22.  —  Tripolar  mitotic  figure  from        (Fig.    22).      Figure    23    shows  a  multi- 
nurse  in  pollen  sac  of  Magnolia.  ^^  spmdle  from 

The   spindle   arises  in  many  Protozoa  and 

Protophyta  within  the  nuclear  membrane.     Its 

origin  within  the  nucleus  has  been  described  in 

certain  plants  such  as  Erysiphe  and  Fucus  (Fig. 

24).     Until  recently  the  aster  was  held  to  be  as 

constant  and  permanent  a  feature  of  the  cell  as 

the  chromatin.      Recent  evidence   makes   this 

quite  unlikely.    The  aster  with  its  rays,  archo- 
plasm and  centriole,  may  as  completely  vanish 

as  the  spindle,  to  reappear  when  mitosis  again 

ensues. 

This    complex    nuclear    activity    is    usually 

accompanied   by  a  division  of  the   cytoplasm 

of  the  cell.     It  occurs,  however,  in  certain  cells 

where  there  seems  to  be  a  demand 
for  greater  nuclear  surface  without 
cytoplasmic  division.  This  results  in 
multinuclear  cells.  We  have  examples 
of  this  in  the  giant  marrow  cells  and  in 
the  male  nurse  cells  of  some  plants. 
In  the  nurse  cells  of  the  anthers  of 
Magnolia,  for  instance,  there  is  an 

FIG.  24.— Mitotic  figure  in  Erysiphe.  Spin-  intermediate  example,  where  we  some- 
die  formed  inside  of  nuclear  membrane  times  fin(J  mitosis  with  Cytoplasmic 
(n.m.)  After  HARPER.  .  .  J  r 

division,  and  at  other  times  without 
cytoplasmic  division  (see  Fig.  383). 

An  Example  of  a  Mitosis  without  Centrosomes.  —  For  this  mitosis  let 


23. —  v-ancer  cell  from 
man.  Multiple  division  by 
many  centers. 


MITOSIS  27 

us  take  that  found  in  the  root-tip  cells  of  a  hyacinth.  The  cells  are 
six-sided,  nearly  cubical  bodies,  fitted  to  form  concentric  layers  around 
a  central  row  of  slightly  larger  cells.  The  cells  are  of  an  average 
size  for  plant  cells.  They  divide  constantly  in  a  plane  usually  at  right 
angles  to  the  length  and  direction  of  growth  of  the  root.  This  division 
and  the  subsequent  elongating  of  the  newly  formed  cells  results  in  the 
lengthening  of  the  root.  Examples  taken  about  10-45  ce^s  back  from 
the  apex  of  the  root  will  form 
the  basis  of  our  description  and 
are  represented  by  Figures  25  to 
36. 

A  cell  while  resting  between 
divisions  (Fig.  25)  possesses  a 
dense  cytoplasm,  of  apparently 
alveolar  formation,  which  at  first 
sight  is  apt  to  give  one  the  im- 
pression of  a  reticulum.  Several 
vacuoles  of  large  size  are  present 
in  the  cytoplasm,  and  these  in- 
crease in  size  with  the  age  of 
the  cell.  No  especial  organs  are  FlG'  25'~ 
to  be  seen  in  the  cytoplasm  except 
an  occasional  vacuole  containing  spindle-like  crystals.  The  nucleus  is 
large,  of  a  diameter  over  two  thirds  the  average  diameter  of  the  cell, 
and  round  or  slightly  compressed  in  outline  in  the  narrower  cells.  It  is 
surrounded  by  a  well-developed  nuclear  membrane. 

The  karyoplasm  is  dense  and  is  composed  of  two  visible  substances,  a 
deeply  staining  chromatin  and  a  semitransparent  linin.  The  chromatin 
is  plainly  seen  to  be  divided  into  many  small  portions  equally  spaced 
through  the  karyoplasm  and  connected  with  one  another  by  strands  of 
the  linin.  Resting  in  the  mesh-like  karyoplasm  are  to  be  seen  one  or 
more  nucleoli.  If  a  single  body,  the  nucleolus  is  found  nearly  in  the 
center  of  the  nucleus;  if  two  or  more  exist  in  the  same  nucleus,  they 
occupy  the  centers  of  portions  of  the  karyoplasm  proportionate  in  size 
to  themselves.  Each  nucleolus  always  lies  in  the  center  of  a  vacuole. 
It  always  lies  in  the  center  of  this  space  and  never  rests  against  the  side, 
thus  leaving  a  clear  zone  around  its  body  and  between  its  own  substance 
and  that  of  the  chromatin-linin  network.  This  indicates  that  the  vacu- 
ole is  occupied  during  life  by  a  more  or  less  solid  substance  of  sufficient 
density  to  prevent  the  nucleolus  from  moving  about.  Besides  the  vacu- 
oles which  contain  nucleoli,  other  spaces  apparently  without  nucleoli  are 
found ;  these  are  always  smaller  than  the  others  and  at  first  sight  con- 
tain only  the  clear  substance.  A  closer  examination,  however,  reveals 


28 


HISTOLOGY 


central  bodies  in  the  larger  of  these  spaces,  and  these  bodies  have  a 
smoky  and  transparent  appearance.  The  very  smallest  spaces  contain 
nothing  at  all  that  can  be  seen  with  the  microscope. 

The  nucleoli  in  nearly  all  cases  contain  vacuoles  in  their  own  sub- 
stances, these  vacuoles  being  filled  with  a  substance  which  does  not  stain 
deeply,  but  to  the  same  degree  as  the  bodies  in  the  smaller  karyoplasmic 
spaces  described  above.  These  nucleolar  vacuoles  vary  in  size  and 
number;  they  increase  in  both  as  the  cell  grows  older.  The  clear  fluid 
or  substance  which  fills  all  the  rest  of  the  nucleus  is  a  decidedly  visible 
body  in  the  cell  we  are  studying.  Although  sufficiently  clear  and  free 
from  staining  power  to  permit  of  our  seeing  all  the  other  organs  clearly, 
yet  it  is  distinctly  visible  in  cells  fixed  in  Flemming's  fluid.  Its  density 
or  consistency  is  not  known  by  any  visible  feature  except  perhaps  the 
fact  that  by  coagulation  it  supports  some  of  the  parts  and  organs  of  the 
nucleus.  All  the  substances  described  are  surrounded  by  the  nuclear 
membrane,  which  forms  a  complete  covering  to  the  entire  nucleus.  This 
membrane  has  an  appreciable  thickness  and  is  homogeneous  in  struD- 
ture.  It  has  considerable  staining  power,  more  than  the  linin  and  less 
than  the  nucleoli. 

The  first  evidences  of  an  approaching  division  are  seen  best  in  the 
chro matin.  Its  particles  appear  blacker  and  larger,  and  a  close  inspec- 
tion shows  them  to  be  more  irreg- 
ular in  outline  and  to  have  ar- 
ranged themselves  in  incomplete 
rows  in  various  parts  of  the  kary- 
oplasm,  thus  altering  the  granular 
structure  to  a  thread-like  appear- 
ance. A  slight  enlargement  of  the 
nuclei,  the  appearance  of  larger 
vacuoles  and  more  of  them  in  the 
nucleolus,  which  becomes  irregu- 
lar in  outline,  and  the  growth  and 
complete  alignment  of  the  chro- 
matin  particles  into  threads  now 
cause  the  stage  to  become  easily 
identified  and  found  among  the 
cells  (Fig.  26).  The  chromatin  at 
this  stage  has  formed  a  complete  and  apparently  continuous  thread, 
called  the  spireme,  in  the  meshes  of  which  the  smaller  and  much  dis- 
torted nucleolus  is  still  seen.  This  thread  is  not  only  complete  but  its 
ends  are  joined,  forming  a  Gordian  knot  which  is  usually  broken  or 
disturbed  by  the  process  of  section  cutting.  It  is  of  considerable 
length. 


FIG.  26. — -Hyacinth  root -tip  cell  showing  first 
signs  of  an  approaching  mitotic  division. 
Chromatin  gathering  in  skein  or  spireme. 


MITOSIS 


FIG.  27.  — Hyacinth  root-tip  cell  with  spireme 
formed.     Nucleolus  dissolving. 


Next  to  take  place  is  a  shortening  of  the  spireme  and  a  thickening 

of  its  diameter,  accompanied  by  a  merging  or  blending  of  the  constituent 

chromatin  granules  into  each  other 

so   that  the   spireme    assumes  a 

smooth,  wire-like  appearance  (Fig. 

27).    Other  phenomena  soon  take 

place.     Two  clusters  of  transpar- 

ent filaments  appear  in  the  cyto- 

plasm, one  on  either  side  of  the 

nucleus  and  opposite  each  other. 
Each  cluster  consists  of  many 

fibrils  of  an  achromatic  substance 

(Fig.  28).   These  fibrils  are  scat- 

tered evenly  and  closely  on  the 

nuclear    membrane,     about    one 

fourth    of    whose    surface     they 

cover.      They  reach    from   there 

outward  toward  a  common  center  in  the   cytoplasm,  at  which  they 

converge.    Thus   each    cluster   of   fibers   forms   a  short,   thick   cone, 

placed   opposite  to   its  neighbor,  and  the   two  together  form  an  in- 

complete diamond-  shaped  outline, 
which  is  called  the  spindle  when  a 
little  further  developed.  Shortly 
after  the  spindle  has  begun  to 
appear,  the  nuclear  membrane 
begins  to  dissolve,  becoming  thin- 
ner, and  finally  disappearing  at 
one  or  more  points,  but  without 
losing  at  any  time  its  rigid  curved 
outline.  It  apparently  dissolves  in 
situ,  and  when  once  started,  the 
breaches  rapidly  widen  until  all 
the  membrane  is  gone.  Its  points 
of  first  disappearance  art  usually, 
but  not  necessarily,  at  the  poles 
as  indicated  by  the  spindle. 

Shortly  after  the  membrane 
has  gone  at  these  poles  the  spindle 
fibers  grow  into  the  nucleus  and 
touch  the  chromatin  spireme.  The 


FIG.  28. — Hyacinth  root-tip  cell  with  spindle 
forming  and  nuclear  membrane  dissolved  at 
two  points.  Nucleolus  appears  larger,  but 
is  smaller  in  bulk,  and  apparent  size  is  due 
to  vacuoles. 


nucleus    then    develops    into    the 

somewhat  more  frequently  seen  stage   pictured    in  Figure   29.     Here 
three  changes  are  apparent.     The   nuclear   membrane  is  completely 


HISTOLOGY 


gone,  the  nucleolus  is  gone,  and  the 


spireme  is  compressed  into  an  oval 
to  round  mass,  lying  at  right  angles 
to  the  spindle  and  the  plane  of  the 
equator,  halfway  from  pole  to 
pole. 

This  round  mass,  which  is  a  flat 
disk  in  the  dividing  cells  of  some 
other  plants  and  animals,  is  still 
the  continuous  spireme  noted  be- 
fore; in  its  later  stages  several 
breaks  may  be  seen  with  the  loose 
ends  springing  up  from  the  main 
body.  The  spindle  fibers,  which 
are  now  developed  into  longer 
cones,  touch  the  equatorial  mass 
of  chromatin  and  appear  to  have 
FIG.  29. -Hyacinth  root-tip  ceil  with  spireme  established  a  continuity  through 

entire  but  ready  to  break  into  the  chromo-      its  meshes,  SO  that  a  fiber  Starting 
somes.    Spindle  formed.    Spireme  appears  to         .  i  without    hrpak 

be  divided  owing  to  process  of  sectioning.  at    OnC    POle    IUnS    wltnOut     OTQ3.K. 

through  the  spireme  to  the  oppo- 
site pole.  The  breaks  observed  in  the  spireme  (not  shown  in  Fig.  29) 
increase  until  the  whole  spireme 
has  been  divided  into  a  number 
of  equal  parts.  There  are  twelve 
in  the  case  of  the  hyacinth  root- 
tip.  These  portions  of  the  spireme, 
known  as  chromosomes,  quickly 
move  into  one  of  the  most  com- 
monly seen  figures  as  pictured  in 
Figure  30.  Here  the  chromosomes 
are  scattered  as  by  the  snap  and 
momentum  of  their  breaking,  and 
yet  two  facts  must  be  noticed; 
some  part  of  each  one  is  still  re- 
tained in  the  equatorial  plane  of 
the  spindle,  and  such  parts  of  the 
chromosomes  as  have  moved  from 
this  plane  lie  usually  among  and 
parallel  to  the  fibers  of  the  spindle. 
The  wandering  portion  usually 
consists  of  a  free  end  of  a  chromo- 
some. 

Ordinarily  the  wandering  ends  are  found  on  each  side  of  the  equa- 


FlG.  30.  —  Hyacinth  root-tip  cell.     Spireme 
newly  broken  into  the  chromosomes. 


MITOSIS 


FIG.  31. — Hyacinth  root -tip  cell.  Chromosomes 
doubled  and  with  their  points  of  first  division 
in  the  equatorial  plate. 


torial  region,  but  in  many  cases  great  indifference  is  shown  in  their 

distribution,  and  it  may  even  be  that  all  ends  reach  toward  the  same 

pole,  leaving  the  other  side  empty 

of  chromatin. 

The  change  from  the  present 

stage  as  seen  in  Figure  30  to  Fig- 
ure  31    is   marked  by  one  very 

noticeable  fact  to  which  all  else  is 

subsidiary.    This    is    that    every 

chromosome    as   of    one    accord 

moves  so  that  the  exact  middle 

of  its  length  will  lie  directly  in  the 

equatorial  plane.     As  an  accom- 
paniment to  this,  its  ends  are  apt 

to  straighten  out  in  a  variety  of 

directions,  so  that  a  definite  though 

irregular  horseshoe-shaped   loop, 

much   elongated,    is   usually   the 

result. 

While    this    is    being    done, 

another   very   important    change 

takes  place.     Each  chromosome  splits  for  its  whole  length  into  two 

halves,  the  halves  remaining  at- 
tached for  all  their  length  by  an 
achromatic  substance  which  per- 
mits of  their  dual  structure  being 
seen.  This  stage  figured  in  Fig- 
ure 31  is  rare.  It  is  very  quickly 
followed  by  the  stage  seen  in 
Figure  32,  where  it  is  quite  evi- 
dent that  each  dual  loop  is  being 
pulled  apart  by  fibers  attached 
to  the  apices,  one  fiber  reaching 
from  a  pole  to  one  half  of  each 
loop,  while  another  fiber  from  the 
opposite  pole  is  attached  to  the 
apex  of  each  of  the  other  daughter 
loops.  Thus,  when  these  fibers 
contract,  the  daughter  chromo- 

FIG.  32.— Hyacinth  root-tip  cell.   Division  of     somes  are  well  apart  (Fig.  33). 

the  chromosomes.  ,_.  •  i_  j          ,      •    / 

They    move   with    ends    straight 
out  and  form  a  common  and  characteristic  figure. 

The  question  naturally  arises  as  to  the  motive  power  involved  in  this. 


HISTOLOGY 


transportation  of  chromatin. 


The  chromosomes  seem  to  possess  the 
power  of  independent  motion,  as 
witnessed  by  their  movements  in 
Figures  30  and  31;  when  pulled 
apart  as  in  33.  However,  the  very 
probable  and  entirely  possible  mo- 
tive power  is  the  contracting  fibril. 
In  Figure  33  the  fibrils  between 
these  two  masses  of  separating 
daughter  chromosomes  show  us 
that  not  all  the  fibrils  of  the  original 
spindle  were  employed  to  draw  the 
parent  chromosomes  apart.  Con- 
sequently we  are  able  to  distin- 
guish between  the  two  kinds,  the 
remaining  fibrils  being  called  true 
spindle  fibrils,  and  those  used  in 
FIG.  33. -Hyacinth  root-tip  cell,  chromo-  the  division  mantle  fibrils,  be- 

somes  reaching  the  spindle  poles.     Spindle       caUSC  they  are  placed  On  the  OUtCI 

surface  of  the  spindle. 

The  spindle,  whose  form  and  poles  were  so  perfectly  seen  in  Figures 
29,  30,  31,  and  32,  is  now,  for  the 
first  time,  seen  clearly  at  the  equa- 
tor from  which  the  chromatin  has 
moved.  This  leaves  the  equator 
free  but  obscures  the  poles,  which 
will  not  be  seen  again  because 
the  chromatin  is  so  thick  and 
dark.  The  spindle  as  seen  in  this 
stage  is  composed  of  true  spindle 
fibers  only,  as  the  mantle  fibers 
were  withdrawn  while  dragging 
the  chromosomes  to  the  poles. 

Two  changes  mark  the  next 
stage  which  is  represented  in  Fig- 
ure 34.  First,  the  chromosomes 
have  shortened  and  thickened  and 
become  compacted  into  a  denser 
mass  than  at  any  other  time  in  the 


whole  process  of  division.  They 
have  partly  lost  their  individual- 
ity by  blending  together  as  though 
being  of  wax  which  had  been  subjected  to  heat. 


FIG.  34.  —  Hyacinth  root -tip  cell.  Massing  of 
the  newly  divided  chromosomes.  Beginning 
of  a  division  in  the  cell-plate. 


Secondly,  there  has 


MITOSIS 


33 


appeared  in  each  spindle  fiber  at  the  equator  of  the  spindle  a  small 
thickening.    These  lie  in  the  same 
plane  and  form  the  cell-plate. 

In  Figure  35  one  can  easily  see 
that  the  coalescing  mass  of  chromo- 
somes has  become  surrounded  by 
a  nuclear  membrane  of  somewhat 
peculiar  shape,  while  the  chromatin 
itself  has  assumed  a  spireme-like 
shape  which  in  proportion  and  tex- 
ture much  resembles  that  of  Figure 
27.  The  whole  series  of  changes 
through  which  the  chromatin  goes 
in  this  part  of  the  division  process 
has  been  compared  to  a  reversal  of 
the  earlier  changes.  It  is  so  to  a 
very  limited  degree  and  with  re- 
gard to  the  chromatin  only.  Figure 
35  also  shows  an  increase  in  the  row 


FIG  35.  —  Hyacinth  root-tip  cell.  The  nu- 
cleus divided  and  the  daughter  nuclei  re- 
forming. Widening  and  further  cutting  off 
of  the  cell-plate. 


of  dots  which  have  become  stronger 

and  formed  the  cell-plate,  which  is 

the  plane  through  which  the  two  daughter  cells  are  shortly  to  separate. 

Our  last  Figure  36  shows  the 
two  daughter  cells  practically  sep- 
arate and  complete;  the  nucleolus 
has  suddenly  reappeared ;  the  chro- 
matin is  nearer  to  that  seen  in 
Figure  26  and  can  easily  be  traced 
to  that  stage  or  to  the  stage  shown 
in  Figure  25  or  the  resting  cell.  The 
nuclei  are  enlarged  and  rounded 
in  outline  and  the  achromatic  fi- 
brils have  all  disappeared.  It  but 
remains  for  the  two  daughter  cells 
to  grow  in  size  and  then  to  begin 
the  division  cycle  over  again. 

An  Example  of  Mitosis  with 
Centrosomes.  — We  select  the  ovum 
of  Unio  for  the  demonstration  of 
mitosis  with  a  complete  achromatic 
figure  because  of  its  availability. 
The  ova  are  found  from  the  early 

days  of  May  to  the  late  days  of  June  in  various  stages  of  segmentation. 


FIG.  36. — The  two  daughter  cells  of  hyacinth 
root-tip.     Reappearance  of  the  nucleoli. 


34 


HISTOLOGY 


The  first  and  second  segmentation  stages  afford  the  best  demonstra- 
tion objects.    The  nucleus  of  these  blastomeres  is,  in  its  resting  stage, 

about  fifty  micra  in  diameter  with  an 
oval  to  rounded  contour.  The  karyo- 
somes  and  plasmosomes  stain  deeply 
and  are  variable  in  number;  usually 
three  or  four  are  present.  The  largest 
one  is  generally  a  deeply  staining  body 
with  a  vacuole  of  lighter  staining  ma- 
terial through  which  are  seen  granular 
strands  of  the  chromatic  material  (Fig. 

37)- 

The  chromatin  is  distributed  as  a 
series  of  fine  particles  throughout  the 
nucleus  and  is  supported  by  a  retic- 
FIG.  37.  — Beginning  of  fourth  cleavage  di-   ulum   of    tough,   non-staining    linin 


visible  by  careful  staining  with  eosin. 

As  in  many  other  resting  cells,  the  strands  of  linin  tend  to  start  radially 
from  the  largest  nucleolus  and  stretch  irregularly  to  the  periphery  of 
the  nucleus  as  a  network.  The  nuclear  membrane  is  sharp  and  well 
developed.  It  stains  with  most  dyes. 

The  resting  nucleus  shows  no  trace  of  a  centrosome  inside  of  it  or 
outside  in  the  cytoplasm.  The  first  sign  of  an  approaching  division, 
besides  an  increase  in  the  staining  power  of  the  chromatin,  is  the  sudden 
appearance  of  a  centrosome  on  each  side  of  the  nucleus.  These  struc- 
tures are  in  the  cytoplasm,  but  rest 
closely  against  the  nucleus.  Each 
centrosome  consists  of  a  large  centri- 
ole  lying  in  a  very  small  centrosphere 
from  which  long,  delicate  rays  pass 
out  into  the  cytoplasm. 

At  this  time  the  linin  disappears 
and  the  chromatin  begins  to  gather 
into  larger  masses  (Fig.  38).  The 
nucleolus  often,  but  not  always, 
shows  much  vacuolation.  The  cen- 
trosomes  grow  in  size  and  apparent 
strength. 

Figure  39  shows  the  next  impor- 
tant step.    The  chromatin  has  formed 
a  very  long  and  thin  spireme,  some  parts  of  which  are  thicker  than  others. 
The  nucleolus  is  very  much  smaller,  and  the  centrosomes  are  beginning 


FIG.  38.  — A  slightly  later  stage  than  in 
last  figure.  Chromatin  gathering.  But 
one  centrosome  shown  in  this  section. 


MITOSIS 


35 


to  move  apart.  As  they  move  they  seem  to  leave  a  vacant,  cone-shaped 
area  between  themselves  and  the  nuclear  membrane.  The  outer  bound- 
ary of  this  space  seems  to  be  the 
straight  rays  from  the  centro- 
some,  which  are  very  well  devel- 
oped and  are  numerous  enough 
to  form  a  continuous  boundary 
(Fig.  39).  A  new  and  weaker 
set  of  radiating  fibrils  appear  in 
this  space,  and  reach  from  near 
the  centriole  to  the  nuclear 
membrane,  upon  which  they 
appear  to  have  some  destructive 
influence.  This  membrane  be- 
gins to  curl  where  so  influenced, 
and  in  the  next  figure  (40)  it  is 
shown  as  very  much  degener- 


ated. 

This  figure  also  shows  the 
chromatin  spireme  broken  up 
into  a  certain  number  of  long, 
bent  chromosomes,  which  are 
position  which  they  must  occupy  before  they  can  be  drawn  apart 


FIG.  39.  —  Later  stage  than  Fig.  38.  Irregular 
spireme,  centrosomes  moving  apart  and  nuclear 
membrane  beginning  to  fail  where  touched  by 
the  forming  spindle  fibrils. 

not   yet   arranged    in    the    equatorial 


-— --r     i-  •    ••  -"— ' — 

FIG.  40.  — Later  stage  of  this  mitosis.    Nuclear  membrane  nearly  gone.    Spireme  breaking  to 
form  chromosomes.     Spindle  fibrils  growing  towards  each  other.     Nucleolus  almost  gone. 

in  longitudinal  halves.    The  spindle  fibrils  which  were  so  weakly  devel- 
oped in  Figure  39  are  here  seen  to  be  more  strongly  developed  than 


HISTOLOGY 


FlG.  41.  —  Same  process  at  time  of  formation  of  equatorial 
plate  of  chromosomes. 


the  astral  fibrils.    These  latter  are  very  long  and  stretch  out  into  the 

cytoplasm  almost  to  the  cell-wall  in  some  places.    The  chromosomes  are 

mossy  at  this  time 
and  the  nucleolus 
sometimes  persists  as 
it  has  done  in  our 
example. 

Figure  41  shows 
the  figure  completed 
and  ready  for  the 
division  of  the  chro- 
matin.  The  spindle 
fibrils  are  at  their 
best  development, 
and  some  of  them 
can  plainly  be  seen 
to  have  become  at- 
tached to  opposite 
sides  of  the  chromo- 
somes and  be  pull- 
ing them  apart.  The 

chromosomes  are  shorter  and  smoother  than  they  were  in  the  preceding 

stage.    They  are  bent  or  V-shaped  rods  which   are  first  split  at  their 

apex.     It   can   be    seen 

in   this   figure   that   the 

strain  on  the  spindle  has 

caused  a  sinking  in  of 

the  whole  surface  at  the 

point  where  this  strain 

is  greatest. 

A  peripheral  layer  of 

the  cell  is  left  in  its  origi- 
nal position.    The  next 

figure  (42)  represents  a 

stage,  subsequent  to  the 

last,  in  which  the  chro- 
mosomes have  been 

drawn  apart.    The  form 

is   well   shown.     As  .in 

the  hyacinth  figure,  the 

spindle  fibrils  are  shown 

between  the  parting  groups  of  chromosomes,   while  the  fibrils  which 

are    seen    between    the    chromosome    groups    and    the    centrosomes 


FIG.  42.  —  A  cell  of  the  same  kind  showing  separation  of 
the  chromosomes. 


AMITOSIS 


37 


must   represent   both    kinds  of   spindle    fibrils,   the    rigid    and   con- 
tractile fibrils  of  the  achromatic  division  figure. 

Figure  43  shows  the 
reforming  daughter 
nuclei.  The  vestiges 
of  the  spindle  and  the 
process  of  division  of 
the  cytoplasmic  body 
are  both  well  shown. 
The  nucleolus  is  slow 
to  reappear  and  all 
traces  of  the  centro- 
some  have  disappeared. 

Technic. — Flem- 
ming's  fluid  and 
sublimate  are  recom- 
mended for  this  work, 
together  with  the  par- 
affin sectioning  method 
and  iron  haematoxylin 
staining.  There  are  no 
special  methods  other 
than  a  few  of  the  ana- 
line  dyes  to  make  dif- 
ferential stains  of  the 

various     parts     Of     the     FIG.  43. — Division  of  nucleus  almost  completed.    Cell  begin- 
,.    .  ,.  11         /".  ning  to  divide. 

dividing     cell.      Great 

care  is  necessary  not  to  injure  the  delicate  structures  by  any  rough 

usage.    Too  quick  dehydration  will  sometimes  injure  the  specimen. 

LITERATURE 

The  same  general  works  should  be  read  as  were  recommended  for  the  last  chapters. 
LILLIE,  F.  R.     "  Organization  of  the  Egg  of  Unio,"  etc.,  Journ.  Morph.,  Vol.  xvii. 


AMITOSIS 

Another  kind  of  cell  division  is  found  in  which  the  complex  processes 
studied  in  mitosis  are  not  present,  and  the  cell  divides  by  a  series  of  auto- 
constrictions  of  first  the  nucleolus,  then  the  nucleus,  and  lastly  the  cyto- 
plasmic body.  This  is  known  as  direct  or  amitotic  division.  Strangely 
enough  this  method  corresponds  almost  exactly  with  Remak's  descrip- 
tion of  cell  division  when  he  first  "  discovered"  and  figured  it  upon  very 


HISTOLOGY 


slender  observation  in  the  chick.  His  observations,  however,  were 
made  on  tissues  that  divided  by  mitosis,  and  he  probably  mistook  some 
later  phases  of  this  process  for  a  process  superficially  comparable  to 
amitosis. 

Amitosis  usually  begins  in  a  cell  by  the  elongation  of  the  nucleolus  at 
right  angles  to  the  future  plane  of  division  of  the  cell.  A  constriction 
of  the  middle  of  this  organ  then  proceeds,  not  as  though  some  power  was 
cutting  it  in  two  as  a  band  or  string  would  do,  but  more  as  though  the 
two  ends  were  being  pulled  apart  and  the  middle  was  thinning  out  to 
the  breaking  point.  The  two  daughter  nucleoli  then  move  apart  to  posi- 
tions in  the  approximate  centers  of  the  two  future  daughter  cells,  and  the 
nucleus  is  ready  to  divide.  Frequently  the  daughter  nucleoli  begin  to 
elongate  as  though  for  another  division  before  the  mother  nucleus  has 
even  begun  to  divide. 

In  other  cases  the  nucleus  acquires  the  two  daughter  nucleoli  not  by 
the  splitting  of  the  old  one  but  by  the  growth  of  a  new  one  in  situ  at  the 
opposite  side  of  the  nucleus  from  the  original  nucleolus.  This  method 
often  results  in  a  splitting  of  the  nucleus  before  the  new  nucleolus  is  as 
large  as  the  other,  and  one  of  the  new  cells  is  then  smaller  than  its  sister- 
cell. 

The  division  of  the  nucleus,  subsequent  to  that  of  the  nucleolus, 
may  be  done  in  one  of  three  ways,  the  first  and  second  of  which  are  much 
alike.  It  may  divide  by  a  constriction  of  its  body,  as  in  the  case  of  the 
nucleolus :  this,  however,  is  rare.  Oftener  it  forms  a  plate  at  the  plane 

of  division,  and  this  becoming 
double,  the  two  plates  separate, 
showing  flat  and  parallel  surfaces 
where  the  separation  took  place. 
The  third  method  of  nuclear  divi- 
sion, recently  suggested  by  Child, 
1907,  from  numerous  observations 
on  many  forms,  is  performed  by 
the  formation  of  two  new  nuclear 
membranes  inside  the  old  one  and 
around  the  daughter  nuclei.  This 
is  strongly  suggested  by  his  figures, 
one  of  which  is  copied  in  our  Fig- 
ure 44.  The  dissolution  or  absorp- 
tion of  the  old  membrane  would 
then  leave  the  two  daughter  nuclei  free  in  the  cytoplasm  and  separate 
from  one  another. 

The  cell  body  is  last  to  divide  in  amitosis,  and  in  many,  perhaps  the 
majority  of  cases,  it  does  not  do  so  at  all.  Or  it  may  form  a  division 


FIG.  44. —  Amitotic  division  in  the  tissues  of  a 
flat  worm,  Planaria  maculata.   (After  CHILD.) 


AMITOSIS 


FIG.  45.  —  Four  stages  in  the  mitotic  division  of  a  cricket's  egg- 
follicle. 


boundary  like  the  cell-plate,  and  then  never  finish  by  a  complete 
separation.  Most  frequently  the  cell  forms  from  two  to  several  nuclei 
and  goes  no  farther,  dying  when  its  duties  are  finished,  which  may  be 
a  short  time  in  certain  follicle  cells  and  stratified  epithelia,  or  may  be 
as  long  as  the  life  of  the  animal,  as  in  the  case  of  the  muscle  cells. 

The  commonly  received  idea,  at  present,  concerning  amitosis  is  that 
it  is  a  terminal  process  in  the  cell's  life  activities  and  is  a  method  for  secur- 
ing more  nuclear  surface  for  use  in  forced  metabolism  or  secretion.  We 
have  three  well-defined  cases  to  examine,  out  of  the  many  that  could  be 
mentioned,  to  prove  that  this  is  highly  probable. 

First,  the  follicle  cells  of  the  cricket's  ovum  show  an  easily  read 
history  that  can  be  interpreted  in  no  other  way  (Fig.  45).  The  ovary 
of  this  insect,  like 
that  of  many 
others,  is  com- 
posed of  a  num- 
ber of  chains  of 
about  seven  dis- 
tinguishable ova 
each.  In  each 
chain  these  are 

continually  being  added  at  the  top  in  a  small  and  immature  state,  while 
at  the  same  time  they  are  being  discharged  from  the  lower  end  into  the 
reproductive  passages  in  a  large  and  mature  condition.  The  ovum 
grows  many  times  in  bulk  and  surface  area  during  its  descent,  the  great- 
est increase  occurring  in  the  lower  end  of  the  chain.  Each  ovum  is 
covered  with  a  single  layer  of  flat  epithelial  cells,  the  follicle  cells,  that 
have  the  work  to  perform  of  transforming  the  food  materials,  brought 
by  the  blood,  into  yolk  material  and  passing  it  on  to  the  ovum,  which  is 
storing  it  up  for  future  use.  This  layer  of  cells  is  fastened  to  the  ovum 
and  accompanies  it  from  the  beginning  to  its  discharge. 

The  follicle  cells  increase  in  size  during  the  descent  of  the  ovum,  but 
not  at  an  equal  ratio  to  the  increase  of  the  ovum's  surface.  Therefore 
they  increase  in  both  numbers  and  in  size  to  keep  the  follicular  covering 
complete.  The  increase  in  numbers  occurs  only  in  the  upper  end  of 
the  chain,  and  is  done  by  mitotic  divisions  of  the  cells.  When  the 
ovum  is  in  the  lower  portion  of  the  chain,  these  same  cells  divide  by 
amitosis,  which  is  somewhat  incomplete  because  only  the  nucleolus  and 
the  nucleus  divide,  the  cell  body  remaining  intact  in  all  cases  observed. 
The  follicle  cells  increase  in  size  at  this  time,  however,  to  keep  the 
follicle  large  enough  to  cover  the  growing  ovum. 

Amitosis  is  clearly  not  a  means  of  cell  multiplication  in  this  tissue. 
Since  it  occurs,  not  only  in  this  well-defined  case  of  rapid  metabolism, 


40 


HISTOLOGY 


but  also  in  many  others,  we  have  some  right  to  connect  it  with  these 
processes  and  to  infer  that  its  principal  object  is  to  provide  increased 
nuclear  surface  as  quickly  and  with  the  least  expenditure  of  energy 
possible. 

A  second  example  is  described  on  page  90  (see  Fig.  87).  Here, 
again,  the  nucleus  divides  by  mitosis,  while  the  cells  are  increasing  in 
number  in  the  primitive  muscle  regions.  When  the  cells  are  sufficient 
in  number  and  begin  to  form  nbrillae,  the  nuclei  rapidly  multiply  by 
amitosis,  probably  to  afford  more  nuclear  surface  to  the  growing  cell, 
which  is  now  secreting  the  heat-  and  motion- producing  substance  in  in- 
creasing quantities. 

The  third  case  that  we  shall  examine  is  a  very  common  one,  and  is 
found  in  nearly  all  stratified  epithelia,  especially  in  the  higher  vertebrates 

(Fig.  46).  Here,  as  in  the  fol- 
licle cells  of  insects,  the  nucleus 
divides  by  mitosis  to  increase 
the  number  of  cells;  but  it 
changes  to  amitosis,  without  a 
division  of  the  cell  body  in  the 
latter  part  of  the  cell's  life. 
Again,  the  probable  object  is  to 
enlarge  the  nuclear  surface  for 

FIG.  46.  — Amitotic  division  in  the  middle  stratum     increased    metabolism,    the   foi- 

mation  of  keratin  in  this  case. 
Strangely  enough  the  special- 
ized parts  of  this  epithelium  that  produce  certain  oils  and  scents  show 
no  signs  of  amitosis  in  their  later  stages  of  secretion  and  degenera- 
tion (see  Chapter  XX). 

The  above  cases  could  be  multiplied  indefinitely.  As  was  seen  in 
the  preceding  part,  mitosis  is  also  used  in  them  for  other  purposes  than 
growth ;  but  we  are  now  confronted  with  the  question,  can  amitosis  be 
used  for  any  real  growth  purposes? 

Opposed  to  the  preceding  general  ideas  as  to  the  meaning  of  amitosis 
we  find  the  conclusions  of  Child  (1907)  and  others  who,  in  very  recent 
papers,  apparently  show  that  amitosis  occurs  very  extensively  as  a  fac- 
tor in  the  growth  and  regeneration  of  many  young  and  unspecialized 
tissues,  including  even  the  reproductive  tissues.  This  is  described  in 
tissues  from  most  of  the  principal  groups  of  animals. 

The  observations  on  which  these  descriptions  are  based  are  as  yet 
"somewhat  fragmentary"  according  to  the  chief  observer,  Child  (1907). 
And  yet  we  must  recognize  that  if  they  contain  any  truth  at  all  they  must 
contain  a  great  mass  of  facts  that  will  seriously  conflict  with  the  pre- 
vailing ideas.  Meves'  observation  on  the  reproductive  cells  of  the 


A  MI  TO  SIS  41 

salamander  seem  to  support  and  afford  a  firm  foundation  for  Child's 
work. 

We  have,  then,  apparently,  another  kind  of  amitosis  which  cannot 
be  explained  in  the  same  way  as  the  follicle  cell's  divisions  and  other 
terminal  kinds  can.  It  appears  to  be  the  same  process  morphologically, 
but  produced  by  different  conditions  and  leading  to  different  results 
that  we  cannot,  as  yet,  differentiate  from  the  conditions  that  control  the 
first  kinds  of  amitosis  mentioned. 

Technic.  — The  technical  methods  for  the  last  two  sections  are  the 
same  as  for  the  section  on  the  cell.  The  cricket's  egg  follicle  epithelium 
is  best  studied  by  fixing  the  ovaries  in  picro-acetic,  washing  in  alcohol, 
and  then  stripping  the  epithelium  off  with  needles  and  staining  it  with  a 
weak  solution  of  methel  green  and  acid  of  fuchsin.  Great  care  is  neces- 
sary to  avoid  taking  the  tough  egg  membrane  off  with  the  epithelium, 
as  it  takes  the  methel  green  so  strongly  that  nothing  else  can  be  seen 
through  it. 

LITERATURE 

Besides  the  general  works  the  papers  by 
CHILD,  C.  M.     "  Amitosis  in  Moniezia,"  Anal.  Anz.,  Band  XXV,  S.  545,  and  other  papers 

by  this  writer  in  subsequent  numbers. 
CONKLIN,  E.  G.     "Amitosis  in  the  follicle  cells  of  the  cricket,"  BioL  Butt. 


CHAPTER   VI 
EPITHELIUM 

EPITHELIUM  is  a  tissue  whose  cells  line  all  outer  and  inner  surfaces  of 
the  animal  body.  In  this  position  these  cells  are  called  upon  to  make  all 
transfers  of  material  from  the  outer  world  into  the  tissues  of  the  body  and 
from  the  body  to  the  exterior.  Epithelium  must  also  act  as  an  agent  for 
the  transfer  of  sensation  and  for  the  mechanical  protection  of  the  organs 
from  the  rubs  and  knocks  of  the  surrounding  media  and  objects. 

Its  position,  with  its  cells  touching  the  surface  of  the  body,  is  its  chief 
distinction,  and  it  is  only  in  consequence  of  this  position  and  the  duties 
that  accompany  it  that  the  cells  are  modified  into  the  great  variety  of 
structure  that  we  find  among  them  individually  and  collectively. 

These  cells  have  a  strong  polarity,  a  differentiation  into  an  inner  and 
an  outer  surface  or  end.  These  ends  differ  strongly,  according  to  the 
work  that  they  have  to  do,  the  outer  end  usually  being  more  highly  differ- 
entiated than  the  inner.  This  difference  accords  with,  first,  the  physio- 
logical fact  that  the  outer  ends  of  the  cells  are  subject  to  the  greater 
variety  of  conditions  which  determine  their  differentiation;  and  secondly, 
materials  for  the  elaboration  of  excretion  and  secretion  products  are 
taken  from  the  intercellular  fluid  or  blood  by  the  inner  ends  of  the  cells. 
These  substances  are  all  much  alike.  By  a  series  of  processes  this  mate- 
rial is  finally  delivered  at  the  outer  ends  of  the  cells  in  the  form  of  excre- 
tion or  secretion  particles.  The  variety  of  these  elaborated  bodies  gives 
a  greater  variation  to  the  outer  than  to  the  inner  ends  of  the  epithelial 
cells. 

The  sides  are  arranged,  with  rare  exceptions,  for  but  one  purpose, 
that  of  fitting  together  with  those  of  the  surrounding  epithelial  cells. 
The  commonest  shape  of  cell,  in  consequence,  is  that  with  six  sides,  ap- 
proximately conforming  to  the  mathematical  requirements  of  the  occasion. 
The  number  of  sides  is  variable  in  different  epithelia  or  even  in  the 
same  one,  and  owing  to  irregularities  of  arrangement  may  be  more  or 
less  than  six.  Sometimes  there  are  many  sides  in  some  cell  that  is  placed 
in  a  position  where  it  is  surrounded  by  a  large  number  of  others. 

The  cells  of  an  epithelium  are  cemented  together  by  the  sides  more  or 
less  firmly,  but  at  the  point  or  line  where  they  touch  each  other,  at  or  near 

42 


EPITHELIUM 


43 


FlG.  47.  —  Diagrammatic  fig- 
ure of  two  cells  and  the  side 
of  a  third,  t.b.,  terminal  bar 
seen  from  a  lateral  view  on 
the  uncut  side  of  a  cell ;  t.b.2, 
cross  section  of  another  ter- 
minal bar  whose  two  parts 
are  somewhat  separated. 


the  surface,  there  is  developed  a  peculiar  "closing  plate"  or  terminal  bar 
that  serves  to  close  the  whole  layer  of  cells  into  an  impervious  layer  or 
covering  (Fig.  47).  The  terminal  bar  is  prob- 
ably impervious  to  all  gases,  fluids,  and  other 
materials,  and  it  remains  for  the  cell  to  deter- 
mine what  shall  and  what  shall  not  pass  into 
or  out  of  the  body.  This  bar  is  double  in 
section,  as  can  be  seen  when  the  two  parts 
are  separated  by  dissolving  the  cement  sub- 
stance that  holds  them  together  (see  Fig.  47). 
Seen  from  a  surface  view,  the  bar  is  rod- 
shaped,  but  it  is  not  altogether  a  continuous 
structure,  being  rather  a  series  of  closely  set 
granules.  These  granules  are  specializations 
of  the  desmochondria  found  on  the  surface  of 
most  cells,  and  the  cytoplasmic  fibrils  that 
end  in  the  desmochondria  are  also  found 
ending  in  the  closing  plates.  These  same 
desmochondria  are  found  all  over  the  surface 
of  the  stratified  epithelial  cells  of  the  mammals.  Here  the  individual 
desmochondria  that  compose  the  bars  are  entirely  separate  and  give 
the  surface  of  the  cell  the  appearance  that  was  formerly  known  as  the 
prickles  on  the  prickle-cell.  The  desmochondria  that  compose  the  ter- 
minal bars  of  most  cells  of  assimilation  are  particularly  easy  to  see 
individually.  In  all  simple  epithelial  cells  the  terminal  bars,  since  they 
lie  between  all  cells,  are  united  into  a  reticulum,  the  meshes  of  which 
conform  to  the  outlines  of  the  cells.  This  appears  in  a  surface  view  of 
the  epithelium. 

We  must  keep  in  mind  that  these  various  features  of  the  epithelial 
cell,  polarity,  shape  together  with  the  large  differences  in  structure  due 
to  the  differences  in  function  are,  as  is  mentioned  above,  the  result  of 
their  position  on  an  inner  or  outer  surface  of  the  body.  This  idea  can  be 
excellently  understood  by  studying  the  dividing  oosperm  of  the  frog  or 
other  amphibian,  which  will  clearly  show  the  origin  of  epithelium  and 
some  first  causes  of  its  differentiation  from  the  other  cells  of  the  body. 

The  oosperm  is  at  first  a  single  cell  whose  entire  surface  touches  the 
exterior  (Fig.  48,  A}.  There  are  no  inner  surfaces  at  this  time.  A  divi- 
sion into  two,  then  into  four,  and  again  into  eight  cells,  makes  every  cell 
in  the  developing  body  have  an  outer  surface  and  three  sides,  but  no  inner 
surface,  as  the  three  sides  bring  it  to  a  point.  All  cells  are  to  be  considered 
as  epithelial  cells  at  this  time,  although  they  perform  all  the  functions  of 
the  body  (Fig.  48,  B}. 

The  next  few  divisions  of  these  cells  result  in  the  formation  of  some 


44 


HISTOLOGY 


cells  that  do  not,  in  any  way,  touch  the  outer  surface.  These  inner  cells 
have  been  differentiated  apart  from  the  epithelial  cells  which  now  lie 
between  them  and  the  outer  world,  and  where  the  two  touch  is 
found  a  surface  that  is  the  inner  end  of  the  epithelial  cell  (Fig.  48,  C). 
Repeated  divisions  now  reduce  the  size  of  the  cell  in  proportion  to  the 
size  of  the  organism  until  we  find  the  condition  seen  in  Figure  48,  D, 
where  the  epithelial  cells  form  a  flat,  even  sheet  of  tissue  covering  every 
part  of  the  body.  Many  other  cells  have  divided  off  from  the  outer  or 
epithelial  cells  and  have  been  added  to  the  inner  mass  of  cells.  Some 
few  epithelial  cells  (i.e.  cells  touching  the  outer  surface)  have  been 
crowded  out  of  the  outer  row  and  pushed  into  the  inner  mass,  where  they 
can  be  distinguished  by  the  pigment  lying  in  what  was  formerly  their 


FIG.  48.  —  Four  stages  in  the  cleavage  of  the  ovum  of  a  toad,  Hyla  pickeringii. 

outer  end.  Other  inner  cells  have  been  crowded  by  the  inner  growing 
mass  out  into  the  epithelium,  a  rarer  case  than  the  former.  Out  of  all 
this  change  only  such  cells  as  maintain  a  position  on  the  outer  surface 
become  the  epithelial  cells.  Their  use  is  determined  by  their  position, 
not  by  any  structure  or  predetermined  feature  that  we  are  able  to 
demonstrate.  After  the  period  of  differentiation,  when  the  epithelial 
cells  have  attained  their  final  characteristics,  they  can  no  longer  take 
up  the  position  or  function  of  a  mesoderm  cell;  nor  can  the  latter 
become  an  epithelial  cell  in  the  higher  forms.  In  the  lower  forms  these 
exchanges  of  position  or  function  are  often  possible  and  take  place 
during  regeneration. 

Each  epithelial  cell  has  a  flat,  even,  outer  end  that  unites  with  the  ends 
of  its  surrounding  neighbors  in  conforming  to  the  surface  of  the  body. 
The  inner  ends  of  the  cells,  at  first  very  irregular  (Fig.  48,  C),  soon  become 


EPITHELIUM 


45 


aligned  to  form  an  inner  surface  (Fig.  48,  D)  which  later  is  provided  with 
a  complete  membrane  that  lies  between  it  and  the  other  body-cells  lying 
inside  it.  This  is  called  the  basement  membrane,  and  differs  much  in  the 
amount  of  its  development.  It  is  not  always  present,  and  may  be  thick 
and  irregular  or  thin,  tough,  and  smooth,  and  the  cells  sometimes  pro- 
ject through  it  into  the  tissue  beneath.  It  provides  openings  for  nerves 
that  come  from  the  interior  to  end  in  the  epithelium  (see  Fig.  192),  and 
for  lymph  channels,  or  even  for  blood  vessels,  when  it  is  otherwise  so 
dense  that  the  lymph  could  not  pass  through. 

The  relations  of  the  cells  to  the  basement  membrane  are  not  always 
the  same.  The  simplest  formation  is  with  all  the  cells  in  a  row  and  their 
proximal  ends  resting  squarely  on  the  membrane.  Or  they  may  only 
touch  the  membrane  with  one  or  more  processes  (see  Fig.  192).  Lastly, 
some  of  them  may  grow  up  and  away  from  the  membrane  altogether, 
being  supported  by  contact  with  those  that  remain  in  the  original  posi- 
tion. This  is  true  of  the  perceptory  cells  of  the  auditory  or  static  epi- 
thelium of  vertebrates  (see  Fig.  192)  as  well  as  of  some  other  forms. 

This  last  principle,  when  carried  farther,  results  in  many  cells  arising 
from  the  row  of  the  simple  form  of  epithelium  and  lying  in  outer  positions. 
They  remain  attached  to  the 
inner  cells  that  are  on  or  con- 
nected with  the  basement 
membrane.  Such  is  called  a 
stratified  or  multiple  epithe- 
lium, and  is  met  with  in  the 
integument  of  most  vertebrates 
and  some  invertebrate  animals 
(Fig.  49). 

In  the  skin  of  embryonic 
mammals  can  be  seen  splen- 
did series  of  developmental 
stages  of  this  form  of  cov- 
ering. Here  the  epithelium 
starts  as  a  simple  columnar 
form,  a  plain  row  of  cells 
in  section;  in  reality  a  single 


FIG.  49. —  Epithelium  from  the  head  of  Sagitta  hex- 
aptera  to  show  a  simple  stratified  •  epithelium. 
(From  SCHNEIDER.) 


sheet  of  six-sided  cells  covering  every  part  of  the  embryonic  body. 
On  the  umbilical  cord  this  epidermis  begins  to  change  into  a  stratified 
epithelium  at  a  number  of  distinct  points,  one  of  which  is  shown  in  the 
illustration  (Fig.  50).  It  can  be  seen  that  the  cells  that  form  the  outer 
portions  are  not  actually  lifted  out  of  the  basal  layer  and  pushed  outward, 
but  that  the  cells  of  the  basal  layer  divide  by  mitotic  division,  and  that  the 
outer  cells  resulting  from  such  divisions  lie  outside  of  the  basal  layer. 


46 


HISTOLOGY 


They  afterward  divide  one  or  more  times  by  themselves,  this  time  by 
an  amitotic  division,  and  are  then  pushed  farther  outward  by  the  next 
divisions  of  the  cells  in  the  basal  layer.  The  amitotic  divisions  do  not 
appear  in  such  early  stages. 

Thus,  the  basal  layer  remains  where  it  started,  continuing  to  divide 
mitotically,  and  only  such  of  its  daughter  cells  as  can  keep  in  contact  with 
the  basement  membrane  remaining  basal  cells.  All  others,  having  left 
the  basal  layer,  divide  once  or  twice  more  by  an  amitotic  division  and  are 
pushed  continually  outward.  When  a  certain  distance  from  the  basal 
layer,  they  fail  to  get  the  proper  amount  of  nourishment,  and  die.  The 
outer  layer  of  dead  cells  is,  as  can  be  seen,  constantly  accumulating  and 
must  be  as  constantly  reduced  in  order  to  keep  its  volume  at  some  nor- 
mal point.  This  reduction  is  performed  by  the  removal  from  the  outer 


FiG.  50.  — Section  of  a  single  point  of  stratification  on  the  umbilical  cord  of  an  embryo  sheep. 
Mitosis  in  the  basal  layer  and  in  the  underlying  connective  tissue. 

surface  of  as  many  of  the  dead  cells  as  is  necessary,  by  abrasion, 
by  the  shedding  of  layers  of  these  cells,  by  the  solution  and  decay  of 
some  of  them  on  moist  surfaces,  and  in  certain  cases  by  the  pro- 
cesses of  oil  formation  that  result  in  their  degeneration  and  destruction 
when  the  product  is  set  free.  They  are  also  used  by  being  built  up 
into  various  defensive  and  offensive  structures,  as  hairs,  horns,  feathers, 
etc.  (see  Chapter  XX). 

An  epithelium  formed  in  the  above  manner  from  a  simple  or  single 
layered  covering  of  cells  is  known  as  a  stratified  epithelium.  Besides 
being  derived  from  the  simple  form  of  epithelium,  it  usually  shows  a  ves- 
tigial arrangement  of  its  basal  layer  in  a  columnar  or  simple  form.  As 
a  secondary  differentiation  we  may  find  the  outer  layer  of  cells  elongated 
and  placed  in  a  row  so  that  they  look  like  the  columnar  form.  Such  cells 
are  secreting  cells,  and  a  good  example  may  be  found  in  certain  folds  of 
the  conjunctiva  of  the  young  alligator,  where  these  cells  are  engaged  in 


EPITHELIUM 


47 


FIG.  51.  —  Pseudostratified  secreting 
epithelium  from  the  conjunctiva  of 
the  alligator,  b.m.,  basement  mem- 
brane. 


secreting  an  oily  substance  for  lubrication  (Fig.  51).  This  last  modifi- 
cation is  rare,  as  can  be  imagined  when  we  consider  that  all  the  mate- 
rials to  be  used  in  secretion  by  the  outer 
cells  must  be  passed  out  to  them  by  the 
epithelial  cells  that  lie  between  them  and 
the  basement  membrane.  This  calls  into 
play  an  extra,  and  what  nature  seems  to 
consider  a  needless,  effort,  and  one  that 
is  avoided  wherever  it  can  be. 

For  such  reasons  the  secretion  of  ma- 
terials is  nearly  always  confined  to  a 
simple  epithelium,  which,  to  get  the  re- 
quired body  of  cytoplasm,  is  composed 
of  much  elongated  cells,  and  thus  be- 
comes columnar.  The  secretion  may  be 
delivered  in  the  form  of  a  fluid  or  of  granules.  It  may  change  from 
one  to  the  other  after  delivery,  and  before  its  use,  or  it  may  be  trans- 
formed into  gas  at  or  about  the  time  of  delivery. 

The  secretion  is  produced,  sometimes  at  the  expense  of  the  cell,  as 
has  been  mentioned  in  the  case  of  stratified  epithelium.  In  mucous 
and  other  columnar  cells  it  is  produced  at  the  expense  of  the  cell's  distal 
cytoplasm,  which  is  later  regenerated. 

The  outer  edges  of  simple  epithelial  cells  show  many  modifications, 
intended  to  be  of  service  to  them  in  their  activities.     In  other  forms  the 
production  and  delivery  is  steady  and  con- 
stant,  and   the  cell  is  always  in  the  same 
condition  physiologically  and  structurally. 

The  cell-product  is  sometimes  a  hardened 
edge  or  a  continuous  cuticle  which  is  pro- 
duced jointly  by  all  the  surface  cells.  In 
other  cases  it  is  a  striated  border,  which 
may  or  may  not  be  furnished  with  cilia. 
Cilia  are  shown  in  figure  52,  which  repre- 
sents the  digestive  epithelium  of  a  email  ple- 
cypod  mollusk,  Cyclas.  As  can  be  seen  here, 
the  cilia  enter  the  cell  and  converge  in  a 
course  through  the  cytoplasm  until  they 
arrive,  as  a  single  fiber,  at  the  side  of  the 
nucleus. 

Technic. — The  very  simplest  methods  are 
of  the  widest  use  in  studying  this  tissue. 
Two  special  methods  should  be  mentioned.  The  use  of  a  little 
nitrate  of  silver  by  brushing  it  on  the  fresh  tissue  after  washing  with 


FIG.  52. —  Cells  from  the  diges- 
tive tract  of  a  plecypod  mol- 
lusk, Cyclas. 


48  HISTOLOGY 

distilled  water  will  usually  bring  out,  after  exposure  to  sunlight,  the 
outlines  of  the  cells  where  they  come  in  contact  one  with  another. 
This  is  of  great  advantage  in  studying  some  kinds  whose  outlines  are 
indistinct  under  other  circumstances.  The  second  method  is  that  of 
teasing.  The  intercellular  cement  substances  are  easily  dissolved  out  of 
most  epithelia  by  the  use  of  certain  weak  fixatives,  as  33  per  cent  alcohol, 
very  weak  osmic  acid,  and  weak  chromic  acid  (see  LEE).  The  individual 
cells  can  then  be  separated  by  gentle  manipulation  and  examined 
by  themselves.  They  may  be  stained  and  permanently  mounted.  A 
very  useful  but  somewhat  difficult  method  was  used  by  the  writers  to 
study  the  relations  of  certain  epithelia  without  entirely  separating  the 
cells.  The  nasal  epithelium  was  somewhat  softened  and  macerated  in  a 
number  of  media  and  then  treated  with  one  per  cent  osmic  acid  to  harden 
and  preserve  it.  It  was  then  stained  in  several  stains  and  carefully  im- 
bedded in  paraffin ;  sections  were  cut  and  mounted  without  being  affixed 
to  the  slide.  The  dissolving  of  the  paraffin  and  addition  of  the  balsam 
separated  the  cells  enough  to  allow  one  to  easily  see  them  separately 
without  having  their  relations  to  one  another  seriously  disturbed. 

LITERATURE 

General  works  and  papers  by 

ZURSTRASSEN,  O.     "  Uber  die  Mechanik  der  Epithelbildung,"  Verh,  d.  Deut.  Zool.  Gesell., 
1903. 


THE  AMPLIFICATION   OF   BODY   SURFACE 

It  is  through  the  epithelial  cells  lining  a  body  that  its  various  material 
relations  with  the  world  are  established,  especially  the  taking  in  and  the 
giving  off  of  various  substances.  Several  facts  must  be  considered  in 
studying  these  substance  exchanges.  One  is  that  only  a  certain  amount 
of  transfer  per  unit  of  time  can  take  place  through  a  given  unit  of  surface. 
And  in  an  organism  of  high  specialization  this  limitation  is  increased  by 
the  fact  that  there  are  many  more  different  kinds  of  exchange  to  be  per- 
formed than  in  lower  forms ;  and  usually  each  kind  of  work  must  have 
its  own  surface  of  a  particular  character. 

The  first  attempts  to  meet  these  conditions  consist  of  various  gross 
arrangements  of  the  body  surface  which  are  purely  morphological  in 
character  and  serve  to  considerably  enlarge  the  square  surface  of  the 
body.  When  these  are  completed,  however,  there  is  still  not  enough  sur- 
face for  the  body  functions  of  larger  and  more  specialized  animals  to  be 
carried  on.  Some  way  is  necessary  whereby  this  surface  can  be  made  to 
do  more  work  without  further  increasing  the  surface  morphologically. 

What  does  occur  to  bring  this  about  is,  in  reality,  an  increase  in 


AMPLIFICATION  OF  EPITHELIAL   SURFACES  49 

surface.  But  it  is  an  histological  process  that  does  not  increase  the 
morphological  or  primary  surface,  and  all  that  it  needs  is  a  little  greater 
thickness  of  this  primary  surface. 

There  are  three  stages  of  this  process,  resulting  in  three  conditions. 
They  are  corrugation,  evagination,  and  invagination,  using  these  terms 
in  a  special  and  histological  sense. 

The  first  of  these,  corrugation,  is  the  simplest,  and  consists  of  the  throw- 
ing of  an  epithelial  surface  into  a  series  of  parallel  folds  on  the  connect- 
ive tissue  base  (Fig.  53).  This 
folding  is  not  necessarily  vis- 
ible to  the  naked  eye,  and  it 
may  or  may  not  yield  and  be- 
come flat  when  the  surface  is 
stretched. 

Good  examples  of  a  real 
corrugation  are  somewhat  rare 
in  the  adult  organism  owing 
to  the  fact  that  this  process 
is  usually  but  the  beginning 

Of   the   Other   tWO.  FIG.  53.— Diagram  of  a  corrugated  epithelial  surface. 

The  corrugated  side  of  the  foot  in  some  lamellibranch  mollusks  shows 
a  splendid  example  of  corrugation,  with  the  limitation  that  it  is  some- 
times partly  obliterated  by  stretching,  when  the  foot  is  extended.  Sec- 
tions at  right  angles  to  these  corrugations  are  diagrammatically  shown  in 
Figure  53.  It  will  be  noticed  here  that  one  cannot  say  whether  folds 
have  been  thrown  up  from,  or  depressions  have  been  made  in,  the  sur- 
face. A  second  form  of  corrugation  can  be  seen  in  the  embryonic  stages 
of  the  small  intestine  of  vertebrates.  This  stage  is  transitory,  however, 
and  soon  passes  into  the  next  form  of  amplification. 

This  next  form  of  amplification  will  be  either  that  of  evagination  or 
invagination,  and  is  determined  by  the  way  in  which  a  cross  folding  is 
brought  about.  Figure  53  shows  a  simple  corrugation,  and  we  must  now 
imagine  that  this  folding  has  not  sufficiently  increased  the  surface  and 
that  a  second  corrugation  is  to  be  superimposed  upon  the  first  to  increase 
its  surface,  not  directly,  for  it  can  be  seen  in  the  figures  that  the  surface 
is  not  mathematically  increased,  but  by  putting  it  in  a  form  in  which  the 
specialization  can  be  carried  much  farther  than  a  plain  corrugation  could 
be.  A  cross  corrugated  surface  would  not  be  torn  so  easily,  and  the 
lumen  would  allow  fluids  to  pass  more  easily  through  it  than  if  it  were 
lined  with  deep  folds. 

This  second  form  of  amplification  is  produced  by  making  a  series  of 
depressions  at  right  angles  to  the  original  folds  (Fig.  54).  These  may 
be  said  to  determine  the  original  folds  as  depressions  rather  than  ridges, 


HISTOLOGY 


The  result,  then,  is  a  surface  marked  into  a  series  of  elevations  by  two 
series  of  grooves  at  right  angles  to  each  other,  and  with  the  areas  thus 

marked  off  rising  into  a 
set  of  elevations.  These 
elevations  are  known  as 
ei 'aginations,  and  they  may 
be  very  long  and  close  set 
or  fewer  in  number.  They 
are  not  always  formed  by 
two  successive  groovings 
of  the  surface,  but  may 
arise  from  it  simply  as 
outgrowths. 

In  the  case  of  the  ex- 

FIG.  S4.-Diagram  of  an  evaginated  epithelial  surface.         amPle  that  W6  sha11  Study> 

the  villi  of  the  small  intes- 
tine in  some  vertebrates,  the  process  of  formation  of  the  evaginations 
is  approximally  the  same  as  the  purely  theoretical  discussion  outlined 
above  has  indicated,  the  first  amplification  of  the  surface  of  the  lower 
small  intestine  in  man  being  in  the  shape  of  grooves  that  are  roughly 
parallel  and  not  straight,  but  arranged  in  a  sinous  course.  The  division 
of  the  ridges  lying  between  these  grooves  into  papillae  by  a  second  set  of 
grooves  at  an  angle  to  the  first  begins  at  an  early  date,  in  fact,  before  the 
first  grooves  are  fully  formed.  In  the  upper  large  intestine  of  man  this 
process  can  be  best  studied  owing  to  the  fewness  of  the  evaginations,  but 
after  the  villi  are  formed,  they  pass  away  and  are  not  seen  in  the  adult 
organ. 

This  furnishes  an  ontogenetic  case  of  a  process  that  may  also  be 
seen  in  a  taxonomic  series. 
The  early  taxonomic  example 
is  the  adult  intestine  in  many 
fishes,  in  which  the  amplifica- 
tion has  only  proceeded  as  far 
as  a  grooving. 

On  the  whole  this  process 
of  amplification  by  evagination 
is  a  rare  one.  The  mechanical 
and  physiological  advantage 
seems  to  be  all  in  favor  of  the 
third,  which  is  that  of  imagi- 
nation. 

This  process  consists  (theoretically  and  many  times  actually)  of  the 
raising  of  a  series  of  ridges  across  the  surface  of  the  original  folds,  at 


FIG.  55. — Diagram  of  an  invaginated  epithelial  sur- 
face. 


AMPLIFICATION  OF  EPITHELIAL   SURFACES  51 

an  angle  to  these  folds,  which  are  thereby  determined  as  ridges  (Fig.  55). 
This  calls  our  attention  to  the  fact  that  the  folds  in  Figure  53  represent 
either  ridges,  or  grooves,  or  both,  as  the  case  may  be  determined  by  other 
circumstances. 

The  placing  of  these  two  folds  at  an  angle  to  each  other  leaves  a 
series  of  pockets  between  them  which  are  called  imaginations.  As 
both  sets  of  folds  are  formed  at  the  same  time,  we  are  not  accustomed 
to  think  of  them  as  such,  but  to  direct  our  attention  to  the  points  which 
appear  to  be,  and  in  most  cases  are,  depressions  from  the  surface.  If  they 
do  originate  between  the  folds,  they  subsequently  are  extended  by  a  real 
inward  growth  or  invagination. 

A  good  example  of  such  a  structure  may  be  studied  in  the  stomach  of  a 
common  carp.  In  a  fish  of  moderate  or  small  size  the  origin  of  these  in- 
vaginations  can  be  traced  to  longitudinal  corrugations  of  the  surface, 
as  seen  lower  down  in  the  intestine,  and  thus  we  can  realize  the  relations 
of  these  surfaces.  An  examination  with  a  strong  lens  of  these  various 
surfaces  on  freshly  killed  material,  washed  and  treated  with  some  hard- 
ening reagent,  should  follow  or  accompany  the  microscopic  work. 

The  last  specimen  to  be  studied  is  the  duodenum  of  some  small 
mammal  that  has  been  carefully  hardened,  and  bulk-stained,  and  cut. 
This  should  be  studied  in  sections  cut  in  the  vertical  and  horizontal 
planes  as  well  as  one  cut  in  an  oblique  plane. 

We  understand  that  a  section  of  an  organ  is  only  an  image  of  it  in 
one  plane.  For  this  reason  a  perfect  vertical  section  of  either  a  corru- 
gation, or  an  evagination,  or  an  invagination,  cannot  be  distinguished 
from  a  similar  section  of  the  other  two.  We  may  also  see  how  a  section 
of  any  one  of  these  will  reveal  its  true  nature  by  many  slight  departures 
from  a  typical  form.  Also  how  it  may  deceive  one  by  sections  taken  in 
the  valleys  or  through  the  ridges  of  one  or  the  other  of  them. 

In  considering  any  amplification  of  an  epithelial  surface  in  this  work 
it  will  be  designated  as  an  evagination  or  an  invagination,  according  to 
the  relations  which  it  bears  to  its  free  or  distal  surface.  When  the  fold 
moves  proximally,  and  contains  a  morphological  lumen  derived  from 
the  free  surface,  it  will  be  called  an  invagination.  When,  on  the  other 
hand,  it  moves  upward,  inclosing  a  core  of  the  tissues  on  which  it  rested, 
it  will  be  termed  an  evagination.  Thus  the  optic  cup  with  its  stalk  will 
be  considered  as  an  invagination,  although  many  embryologists  speak  of 
it  as  an  evagination. 

Technic.  —  No  special  methods  for  this  part. 

LITERATURE 

Same  as  for  the  last  part. 


52  HISTOLOGY 


GLAND   FORMATION 

Glands  may  be  defined  as  portions  of  epithelial  surface  used  for  the 
secretion  or  excretion  of  some  particular  substance  or  substances  from 
the  body.  This  surface  may  be  small,  in  fact  it  may  consist  of  a  single 
cell,  and  often  does.  It  usually  consists,  however,  of  quite  a  number 
of  cells,  and  there  may  be  several  different  kinds  of  cells  among  them. 

This  surface  may  be  a  part  of  some  general  body  surface,  but  most 
often  it  is  removed  from  the  surface  to  which  it  belongs  by  an  invagina- 
tion.  It  may  be  corrugated  or  evaginated,  but  the  commonest  form  is 
an  invaginated  gland. 

We  shall  study  as  an  example  of  a  unicellular  gland  the  mucous  cell 
found  in  the  digestive  epithelium  of  the  worm  Cerebratulus  (Fig.  56). 


FIG.  56.  —  Portion  of  digestive  epithelium  of  Cerebratulus  lactatus,  showing  a  deep 
mucous  cell,    n,  nucleus  of  mucous  cell,     x  1200. 

This  epithelium  shows,  in  section,  a  straight  row  of  cells  that  are  used, 
evidently,  for  digestive  purposes.  At  intervals  among  this  layer  of  even 
cells  appears  one  which  is  so  large  that  its  body  has  grown  down  out  of 
the  row  and  become  many  times  its  original  bulk.  It  is  forced  to  do  this 
by  the  kind  of  work  that  it  must  perform,  the  secretion  of  mucus. 
This  necessitates  the  large  bulk  of  cell  body,  and  as  the  surface  would  lose 
its  value  as  a  digestive  (and  motor)  surface  were  these  great  cells  to  spread 
the  others  apart  and  wedge  themselves  in  between,  the  large  mass  of 
the  mucous  cell  is  kept  below  and  only  its  distal  end,  with  the  average 
diameter  of  the  undifferentiated  cells,  remains  in  the  row.  The  nu- 
cleus is  placed  in  the  bottom  of  such  a  cell.  See  also  the  mucous  cell 
of  Helix,  pictured  by  Figure  338. 

An  example  of  a  unicellular  gland  that  does  not  descend  into  the  tissue 
below  the  epithelium  of  which  it  is  a  part  can  be  seen  in  the  goblet  cell 
of  the  digestive  epithelium  in  the  intestine  of  any  vertebrate  animal  (see 
Fig.  263). 


ORIGIN  OF  GLANDS 


53 


Multicellular  glands  that  have  not  been  invaginated  are  rare.  Where 
the  unicellular  mucous  glands  are  collected  closely  on  a  surface,  as  in  some 
mollusks,  and  also  where  primary  surfaces  of  the  digestive  tract  are  used 
to  produce  some  special  fluid,  we  have  examples  of  such  surface  glands. 

By  far  the  greater  number  of  multicellular  glands  are  evaginated  from 
the  primary  surface.  The  simplest  way  in  which  this  can  occur  is  as  a 
mere  single  pocket  which  may  be  a  simple  tube-like  depression  called  the 
tubular,  or  a  bag-like  enlargement  called  the  alveolar  type  of  gland. 


FIG.  57.  —  A-G.  Diagram  of  the  different  sorts  of  glands.  A,  unicellular  glands  lying  in  the 
epithelium  and  one  of  them  projecting  out  of  it;  B,  multicellular  gland  not  invaginated; 
C,  simple  saccular  gland;  D,  simple  saccular  gland  with  neck  elongated  into  duct;  E,  com- 
pound tubular  gland  with  individual  ducts  collecting  into  a  single  opening;  F,  compound 
saccular  gland ;  G,  highly  compound  gland  with  one  primary  and  many  secondary  invagina- 
tions.  Only  the  secondary  epithelium  secretes. 

These  simple  glands  are  the  kind  that  usually  produce  the  amplification 
of  a  body  surface  (Fig.  57).  Sometimes  the  lower  part,  or  fundus,  of 
this  simple  gland  does  all  the  active  secretion,  and  the  upper  part  forms 
the  tube  or  duct  that  carries  the  secretion  from  the  fundus  to  the  surface. 
When  the  fundus  becomes  a  highly  differentiated,  saccular  region  on  the 
end  of  a  duct,  it  is  called  an  acinus. 

Many  glands  are  compound  in  that  they  consist  of  two  or  more  acini 
on  the  ends  of  a  branching  duct,  which  acts  as  a  common  carrier  for  their 
secretions.  Compound  tubular  glands  are  to  be  seen  in  the  digestive 
gland  of  the  lobster  and  the  pepsin  glands  of  the  mammalian  stomach. 


54 


HISTOLOGY 


Compound  saccular  glands  appear  in  the  salivary  glands  of  the  mammals, 
of  which  easily  studied  examples  are  to  be  seen  in  the  lower  part  of  the 
tongue. 

Besides  such  compounding  of  the  simple  glands  a  second  sort  of 
amplification  may  appear  in  which  the  walls  of  a  primary  acinus  may 
be  invaginated  into  acini  of  a  different  character.  These  we  shall 
designate  as  the  complex  glands.  A  particularly  good  example  of  such 
a  complex  gland  is  to  be  seen  in  the  stomach  region  or  proventriculus  of 
birds,  of  which  the  pigeon  will  furnish  us  with  a  good  example  (Fig.  57,  G). 
In  this  specimen  a  section  will  show  that  each  of  the  numerous  glands 
opening  into  the  lumen  of  the  proventriculus  is  a  simple  saccular  form 
lined  with  an  epithelium  that  is  different  from  that  of  the  surface  upon 
which  it  opens.  The  exact  use  of  this  primary  gland  epithelium  is  not 
known,  but  judging  from  its  homologies  it  should  be  pepsin  producing. 
Its  staining  reactions  and  general  appearance,  as  well  as  the  fact  that 
the  chief  ferment  of  digestion  is  produced  elsewhere,  would  indicate  that 
this  function  has  been  lost.  Opening  from  all  sides  into  this  primary 
fundus  are  many  smaller,  secondary  invaginations.  These  are  lined  with 
a  totally  different  kind  of  cell  whose  function  has  been  found  to  be  that 
of  producing  hydrochloric  acid.  These  cells  are  represented  in  Figure 
268,  B,  in  Chapter  XV. 

Another  complex  gland  is  also  a  digestive  gland  whose  primary  and 
secondary  invaginations  produce  digestive  ferment  and  hydrochloric 
acid  respectively.  This  is  the  tubular  (usually)  gland  found  in  the 

stomach,  and  we  shall  examine  the 
form  seen  in  the  muskrat  (see  Fig. 
268,  A}.  The  point  to  be  made 
here  is  that  in  the  mammal  acid 
gland  we  have  a  complex  gland 
whose  secondary  elements  are  uni- 
cellular glands  that  have  been 
slightly  retired  from  the  primary 
glands  as  secondary  invaginations. 
The  ducts  of  glands  form  an  in- 
teresting study.  The  duct  is  usu- 
ally an  intercellular  passage  with 
from  three  to  six  cells  forming  its 
walls  at  any  one  transection.  On 
the  one  hand  it  may  become  much 
larger  when  it  carries  off  the  secre- 
tion products  of  very  many  gland  bodies.  In  this  case  it  is  furnished 
with  strong  connective  tissue  coverings  or  even  muscle  layers  to 
strengthen  and  contract  it  (see  Fig.  58).  On  the  other  hand,  the 


FIG.  58.  —  Transverse  section  of  part  of  a 
medium-sized  duct  of  the  cat's  submaxil- 
lary  gland.  X  870. 


SUPPORTING  AND    CONNECTING    TISSUES  55 

gland  duct  may  be  reduced  in  size  until  it  is  an  opening  between  two 
contiguous  cells,  as  is  to  be  seen  in  the  mammalian  liver.  Or  it  may 
be  a  passage  through  the  embracing  arms  of  a  single  cell.  This  is  well 
shown  in  the  earthworm's  nephridium  (see  Chapter  XIX). 

Lastly,  the  terminal  proximal  branches  of  a  duct  enter  often  into  the 
body  of  the  cytoplasm  to  form  the  intracellular  gland  ducts. 


CHAPTER  VII 
THE   SUPPORTING  AND   CONNECTIVE  TISSUES 

PROTOPLASM  as  such  has  not  sufficient  tensile  strength  or  rigidity 
to  enable  the  multicellular  body  to  preserve  its  proper  form.  A  certain 
type  of  cells  is  found  which  are  devoted  to  this  function.  They  fulfill 
it  by  the  formation  of  fibers,  plates,  and  masses  of  material,  either  on  the 
surface  of  their  cytoplasm  or  within  it.  These  extra-cellular  fibers, 
plates,  and  masses  are  made  sufficiently  strong,  rigid,  or  elastic  to  meet 
the  requirements  for  supporting  that  portion  of  the  body  in  which  they 
are  placed.  They  are  controlled  absolutely  in  their  development, 
growth,  and  change  by  the  cells  from  whose  cytoplasm  they  originate. 
We  may  call  them  the  cell  organs  of  support  or,  collectively  with  the 
cells  that  made  them,  the  connective  tissues. 

The  resistance  afforded  by  the  connective  tissues  to  the  body  is  of 
two  kinds :  A  binding  or  connecting  power  secured  by  thin,  strong  fibrils 
in  larger  or  smaller  bundles,  and  a  resistance  to  impact,  shearing,  or 
bending  pressures  met  by  rigid  masses,  shells,  or  rods.  We  can  distin- 
guish in  consequence  two  classes  of  these  tissues :  The  tensile  or  bind- 
ing and  the  rigid  or  supporting  connective  tissues.  It  can  be  seen  that 
all  rigid  connective  tissues  have  some  tensile  strength  and  vice  versa,  but 
yet  the  predominant  character  of  the  tissue  can  be  easily  determined. 
In  some  tissues  the  two  kinds  are  mixed  to  meet  peculiar  conditions. 

Of  each  of  these  tissues  there  are  two  kinds.  In  one  case  the  cell 
organ  of  support  is  an  intra-cellular  organ  and  is  formed  inside  the  cell. 
This  more  primitive  method  is  rare  and  only  occurs  among  the  lower 
forms  of  life.  In  the  second  case  the  cell- organ  is  formed  outside  of 
the  cell,  and  the  same  organ  (fibril,  plate,  or  mass)  is  usually  formed  by 
several  cells  jointly. 

Among  the  vertebrate  animals  the  chemical  constitution  of  the  extra- 
cellular connective  material  is  used  to  classify  the  tissues  into  several 
groups.  As  this  classification  breaks  down  when  extended  into  the  in- 
vertebrate phyla,  we  shall  not  use  it.  For  this  classification  the  reader  is 
referred  to  some  good  medical  histology. 

Adaptability  and  extreme  range  of  variation  are  characteristics  of  the 
connective  tissues,  which  develop  or  change  to  meet  all  kinds  of  require- 
ments in  the  growth,  renewal,  and  regeneration  of  the  organism.  These 

56 


SIMPLE  RIGID   SUPPORTING    TISSUES 


57 


changes  of  the  tissue  are  produced  by  an  increased  formation,  or  a  destruc- 
tion of,  the  connective  materials  under  the  influence  of  the  cells  to  which 
they  belong.  No  tissue  responds  more  quickly  than  connective  tissue 
does  to  a  sudden  development  due  to  exercise.  When  the  muscles 
enlarge  and  grow  stronger  by  practice,  the  tendon  and  the  bone  both  do 
the  same  at  the  same  time  and  at  the  same  proportional  rate  of  speed. 
Technic.  — This  will  be  indicated  under  the  following  parts. 

LITERATURE 

Read  general  works,  especially  the  early  discussion  of  connective  tissues  in  Schneider. 


THE    SUPPORTING    AND    CONNECTIVE    TISSUES  :    SIMPLE   RIGID 

FORMS 

A  Primitive  Form  of  Rigid  Connective-tissue  Cell.  —  "  Leidig's  cell 
of  the  first  order  "  in  a  crustacean,  Homarus.  In  various  parts  of  the 
internal  anatomy  of  the 
lobster  and  nearly  related 
Crustacea  are  placed 
masses  of  supporting  tis- 
sue. These  masses  uphold 
and  support  the  various 
delicate  tissues  about  them 
and  protect  them  from 
the  impact  of  surrounding 
organs.  The  largest  and 
most  regular  of  the  cells 
which  compose  this  tissue 
are  known  as  "  LEIDIG'S 
cells  of  the  first  order," 
named  after  their  discov- 
erer (Fig.  59). 

The  large,  well-formed    FIG  59.  _Connective  tissue  cells  from  the  lobs)^r.  cyt^ 

nucleus  Occupies   the   CCn-         cyto plasmic  mass ;  cyt.p.,  cyioplasmic  processes;  per.,  pe- 
tral     Dart     Of     each      Cell          ripheral  layer  of  cytoplasm  on  which  the  rigid  material  is 
laid  down. 

It  appears  to  lie   outside 

of  the  main  central  body  of  the  cytoplasm.  This  is  due  to  its  extreme 
eccentric  position  in  the  cytoplasm,  the  bulk  of  which  forms  a  dull  gray 
mass  lying  apparently  alongside  of  the  nucleus.  This  cytoplasmic  mass 
shows  in  its  body  a  darkly  staining  area  that  is  round  in  outline  and 
much  smaller  than  the  nucleus.  This  is  probably  the  centrosphere, 
containing  the  centrosome. 


-cyt.  p. 


58  HISTOLOGY 

The  important  feature  of  this  cell  is  the  distribution  of  its  periph- 
eral cytoplasm,  which  is  drawn  out  into  fine -branching  processes  that 
radiate  from  the  central  mass.  The  processes  arrive  at  a  common 
boundary  which  lies  concentrically  about  and  at  some  distance  from  the 
main  cytoplasmic  mass,  making  this  its  center  in  preference  to  the 
nucleus.  The  processes  expand  their  ends  on  this  boundary,  joining 
with  the  expanded  ends  of  the  other  processes.  The  outer  cytoplasmic 
boundary,  so  formed,  is  a  very  thin  shell  or  hollow  sphere  sometimes  a 
little  irregular  in  shape,  and  it  is  about  this  shell  of  cytoplasm  and  by  its 
agency  that  the  rigid  supporting  material  of  the  tissue  is  formed.  The 
amount  and  arrangement  of  this  substance,  which  is  homogeneous  and 
darkly  staining,  is  determined  by  the  mechanical  requirements  to  be  met 
at  this  point.  Figure  59  shows  a  group  of  three  of  these  cells  where 
the  supporting  substance  is  not  as  great  in  amount  or  as  specialized  in 
arrangement  as  in  the  second  and  third  orders  of  LEIDIG'S  cells. 

The  peculiar  arrangement  of  the  cytoplasm,  in  threads  radiating  from 
the  central  mass  and  ending  in  the  peripheral  boundary,  results  in  a  cell 
which  is  not  filled  by  its  cytoplasm.  This  is  explained  when  one  realizes 
what  a  large  and  unwieldy  cell  would  result  if  the  cytoplasm  should  fill 
the  cell  entirely.  The  amount  of  cytoplasm  would  be  excessive,  and  life 
could  not  be  supported  in  the  tissue.  This  peculiar  distribution  of  a 
smaller  amount  fulfills  all  requirements. 

A  body  of  such  cells  is  a  tissue  admirably  adapted  to  its  use.  The 
compact  mass  of  such  rigid  shells  forms  a  framework  which  possesses 
sufficient  rigidity  to  protect  the  delicate  organs  which  it  surrounds  from 
impact  and  shearing  strains  imposed  by  the  heavier  surrounding  organs. 

The  renewal  of  this  tissue  is  easily  seen,  especially  in  the  lobsters 
that  are  about  to  cast  off  their  old  shell  or  have  just  done  so,  as  in  the 
specimen  used  in  this  demonstration.  The  nucleus  divides  by  amitotic 
division  and  the  two  daughter  nuclei  move  to  opposite  sides  of  the  cy- 
toplasmic body.  A  line  of  separation  then  appears,  passing  through  the 
cytoplasmic  body  and  extending  from  periphery  to  periphery  of  the  cell. 
The  resulting  daughter  masses  of  cytoplasm  then  move  apart.  They 
carry  their  nuclei  on  the  side  farthest  from  the  line  of  division.  The 
specific  connective  substance  of  the  cell  is  then  laid  down  in  the  dividing 
septum,  which  thus  becomes  a  new  side  to  each  of  the  daughter  cells  (see 
Fig.  59).  It  is  of  interest  to  note  that,  during  multiplication,  these  cells 
lose  no  part  of  their  functional  power,  but  continue  to  function  as  con- 
nective-tissue elements. 

We  shall  next,  in  pursuance  of  this  idea,  study  the  cells  in  the  growing 
root-tip  of  a  plant,  selecting  that  of  the  chestnut  for  the  purpose. 

In  plants,  nearly  every  cell  in  the  organism  is,  in  addition  to  what  other 
features  it  may  possess,  a  rigid  connective-tissue  cell.  This  is  almost 


SIMPLE  RIGID   SUPPORTING    TISSUES 


59 


axiomatic  and  can  be  easily  demonstrated.  The  development  of  the  cell 
and  its  rigid  connective-tissue  organ  is  shown  to  great  advantage  in  a  longi- 
tudinal section  of  a  growing  root-tip.  We  have  selected  a  root-tip  from 
the  chestnut  seedling.  Young  cells  selected  from  near  the  tip  are  solid 
masses  of  cytoplasm  containing  a  nucleus  (Fig.  60,  A).  They  are  sur- 
rounded by  a  well-defined  cell-wall  of  some  strength  and  thickness.  This 
wall  is  the  connective- tissue  material  and  is  visible  from  the  very  first. 
When  a  little  further  developed,  a  vacuole  appears  in  the  cytoplasm,  and 
later  one  or  more  others  appear  near  the  first.  As  the  cell  grows  larger, 
these  increase  in  size  and  push  the  nucleus  over  to  one  side.  In  an  older 
cell  the  two  or  more  vacuoles  have  broken  through  the  intervening 
walls  of  cytoplasm  and  united  to  form  one  large  vacuole  (Fig.  60,  B). 


FIG.  60.  — A-C.    Three  stages  in  the  development  of  a  plant  cell  as  an  organ  of  rigidity. 

In  some  cases  more  than  one  vacuole  is  left.  At  this  time  it  will  be  seen 
that  the  size  of  the  cell  is  very  much  greater,  especially  in  length. 

The  vacuoles  increase  in  length  until  we  find  a  mere  shell  of  proto- 
plasm lining  the  thick  cell-wall  and  a  larger  mass  of  protoplasm  some- 
where nearer  the  center  (Fig.  60,  C).  This  mass  contains  the  nucleus 
and  is  connected  with  the  peripheral  shell  by  strands  and  bridges  of  cyto- 
plasm. During  life,  a  circulation  of  the  whole  mass  keeps  the  nucleus 
in  touch  with,  and  in  command  of,  its  most  distant  portions  of  cytoplasm. 
Thus  the  peripheral  shell  is  able  to  make  and  preserve  intact  the  impor- 
tant cell-wall,  which  is  the  cell-organ  in  this  case  and  provides  rigidity. 
The  cell  has  enlarged  during  this  development. 

The  same  result,  a  relatively  greater  surface  on  a  given  mass  of 
cytoplasm,  is  thus  attained  as  it  was  attained  in  the  Leidig's  cell  of  the 
lobster.  On  the  other  hand,  one  should  notice  that  there  is  no  renewal, 
that  the  cell  once  formed  goes  through  its  direct  form  of  development  and 


6o 


HISTOLOGY 


then  becomes  fixed  in  its  permanent  form.  This  permanent  form  varies 
in  different  plants,  its  highly  developed  form  being  the  wood-cell,  which  is 
a  cell  in  which  the  cytoplasm  has  developed  the 
cell-wall  out  of  cellulose  and  then  has  added  an- 
other and  more  efficient  material,  xylem,  that,  in 
the  aggregate,  forms  the  ordinary  wood  of  the 
larger  plants  and  trees  (Fig.  61). 

One  other  example  shows  us  a  simple  rigid  con- 
nective-tissue cell  that  is  constructed  to  perform  its 
functions  in  a  slightly  different  way.     The  noto- 
chord  of  the   vertebrates  is  an  organ  that  is  typi- 
cally an  organ  of  rigidity  (Fig.  62).    It  is  composed 
of  cells  that  during  their  development  form  a  single 
large  vacuole  in  their  center  to  secure  a  large  sur- 
face on  a   small   body  of  cytoplasm.    The   outer 
portion  of  this  cytoplasm  secretes  a  shell  of  dense, 
firm  material,  whose  strength,  in  connection  with 
that  of  all  the  other  cells  around  it,  gives  to  the 
FIG.  61.— A-c.   Three    notochord  its  characteristic  firmness.     In  the  pro- 
(F^°sra^uRGER    cess  °^  vacu°tization  the  cytoplasm  is  pushed  from 
after  SCHENCK  and    the  center  to  the  periphery,  where  it  forms  a  shell 
SCHIMPER.)  that  secretes  the  ceU-waU.    This  cell-wall  is  the 

cell-organ  of  rigidity.  The  cytoplasm  inside  of  this  connective-tissue 
shell  is  much  thicker  at  one  point 
than  anywhere  else,  and  the  nu- 
cleus is  located  in  this  thickened 
mass.  This  form  is  a  temporary 
embryonic  one  and  has  no  re- 
newal process. 

It  should  be  noticed  in  all  the 
above  examples  that  the  vacu- 
oles,  which  play  such  an  impor- 
tant part,  are  filled  with  a  fluid 
which,  for  a  better  knowledge  of 
its  use  and  constitution,  will  be 
called  the  cell-sap. 

We  now  turn  our  attention 
to  what  is  an  extremely  primi- 
tive but  comparatively  rare  form 
of  rigid  connective  tissue :  This 
is  the  spicule-forming  cell  of 
certain  sponges  (Fig.  63).  Many  cells  of  the  mesogloea  of  such 
a  sponge  form  in  their  cytoplasm  a  tiny  pointed  rod  of  calcium 


FIG.  62.  —  Notochordal  cells  from  an  embryo 
toad  fish,  Opsanus. 


SIMPLE    TENSILE    CONNECTING    TISSUES 


6l 


carbonate.     This  grows  until  it  gets  too  large  for  the  cell  and  its  end 
sticks  out. 

At  this  time  it  is  used  for  a  rigid  support  for  the  body  of  the  sponge  in 
connection  with  many  other  similar  rods.  The 
spicules  (Fig.  63,  A,  B),  as  these  structures  are 
called,  assume  a  great  variety  of  forms.  (For 
the  supporting  cells  in  certain  integuments, 
see  Chapter  XX on  the  "Integument.") 

Technic. — In  general,  fixatives  containing 
acids,  particularly  acetic  acid,  should  be 
avoided  in  the  preparation  of  connective  tis- 
sues. The  rigid  forms  are  least  damaged, 
especially  when  they  are  chitinous  in  nature. 
Flemming's  fluid  and  sublimate  should  both 
be  used  for  such  examples  as  are  used  in  this 
part.  Sublimate  is  neutral  in  its  action,  and 
is  very  useful  where  it  is  desirable  to  study 
the  cell  structure  as  well  as  a  spicule  or  other  extra-cellular  formation. 
This  sublimate  must  always  be  carefully  removed  by  means  of  iodide 
of  potassium  (see  chapter  on  technic). 


FIG.  63.  —  Developing  spicules 
of  a  sponge.  (From  SCHNEI- 
DER after  A.  MAAS.) 


LITERATURE 

BUTSCHLI,    O.,    1901.      "Einege    Beobachtungen   iiber   Kiesel-    und   Kalknadeln    von 

Spongien,"  Zeits.  f.  Wiss.  Zool.,  Band  LXIX. 
BIDDER,  G.,  1898.     "The  Skeleton  and  Classification  of  Sponges,"  Proc.  of  the  Royal 

Soc.,  London. 
HOLMGREN,  N.,  1902.     "  Uber  das  Verhalten  des  Chitins  und  Epithels  zu  den  unterlie- 

genden  Gewebsarten  bei  Insecten,"  Anat.  Anz.,  Band  XX. 
SCHNEIDER,  K.  C.     "  Lehrbuch  der  Histologie." 


THE   SUPPORTING  AND   CONNECTIVE  TISSUES:    PRIMITIVE  TEN- 
SILE  FORMS 

Examples  of  cells  that  bind  the  parts  of  a  body  together  are  universal. 
The  simplest  form  is  seen  to  greatest  advantage  in  the  embryos  of  all 
animals  before  the  extra-cellular  substance  has  assumed  such  proportions 
as  to  obscure  the  cell  and  its  nucleus.  An  example  is  found  in  the  um- 
bilical cord  of  a  sheep  about  one  quarter  advanced  in  its  intra-uterine 
development  (Fig.  64). 

The  tissue  lying  directly  under  the  external  epithelium  is  composed 
of  a  number  of  cells  spaced  at  somewhat  regular  distances  from  each  other. 
Each  cell  has  its  nucleus  placed  in  the  middle  of  the  cell  body.  This 


62 


HISTOLOGY 


nucleus  is  unspecialized  and  similar  to  the  other  tissue  nuclei  in  its 
neighborhood. 

The  characteristic  feature  is  the  cytoplasm.  This  is  drawn  out  into 
tapering  strands,  of  which  two  to  four  or  five  are  usually  present  in  the 
plane  of  section.  These  arms  of  cytoplasm  meet  the  arms  from  other 
cells  and  form  all  together  a  network  which  fills  the  space  and  holds  every- 
thing together  by  the  union  of  the  arm-like  processes  (Fig.  64). 

As  before  stated,  the  cytoplasm  is  not  in  itself  strong  enough  to 
do  the  work  that  these  tissues  will  be  called  upon  to  do.  The  required 
strength  is  furnished  by  bundles  of  fine  thread-like  bodies  of  great 
strength,  which  appear  about  the  cytoplasmic  processes.  These  threads 
are  formed  by  the  cytoplasm  and  controlled  by  it.  They  extend  from 


FIG.  64. — Part  of  a  section  across  the  umbilical  cord  of  an  embryo  sheep.  Under  the  epithe- 
lium are  seen  the  young  connective-tissue  cells  with  branching  cell  bodies  that  form  a  reticu- 
lum.  These  cells  divide  by  mitosis,  two  stages  of  which  may  be  seen  in  the  figure. 

cell  to  cell  and  by  becoming  united  to  similar  neighboring  bundles  of 
threads  at  various  angles,  they  form  a  very  strong  network  of  tissue. 
They  are  not  strongly  formed  at  this  time  in  the  specimen. 

The  extreme  adaptability  of  such  a  tissue  forms  an  interesting  study 
in  early  embryonic  sections.  The  cells  which  ordinarily  reach  and  hold 
in  all  directions  are,  when  placed  in  some  special  position,  rapidly  modi- 
fied to  meet  any  unusual  strain  from  any  given  direction.  Thus,  close 
to  the  epidermis,  they  flatten  out  in  a  plane  parallel  to  the  surface  to  pro- 
tect the  parts  from  injury  when  stretched  or  punched.  Around  the  blood 
vessels  they  elongate  into  two -armed  spindles,  which  encircle  the  tube  and 
hold  it  while  the  blood  comes  in  waves  and  spurts  against  its  walls. 

The  growth  of  this  tissue  is  performed  by  mitotic  division,  as  shown  in 
two  stages  in  Figure  64.  When  fully  developed,  most  of  it  forms  the 
so-called  lax  connective  tissue  between  muscle  and  skin  and  the  fascia  of 
muscles. 


HIGHER    TENSILE    CONNECTING    TISSUES 


63 


Such  tissue  can  also  be  found  in  many  very  low  animals  and  the  em- 
bryos of  most  invertebrates.  In  vertebrate  embryos,  besides  being  under 
the  epidermis  of  the  umbilical  cord,  it  is  found  under  the  skin  of  the  whole 
body  and  among  all  the  organs. 

Neuroglia,  because  of  the  form  of  its  cells,  might  be  placed  in  this  class 
of  connective  tissues.  Because,  however,  of  its  association  and  common 
origin  with  nervous  tissues,  we  have  placed  it  with  the  nervous  tissues  in 
Chapter  XIII. 

Technic.  —  Carefully  made  sections  fixed  in  most  of  the  ordinary 
fixatives  will  give  good  pictures  of  the  cell  elements.  When  it  is  desired 
to  see  the  extra-cellular  fibrils,  a  fixative  without  acid  must  be  selected. 
Both  paraffin  and  celloidin  sections  should  be  used.  The  stain  can  be  a 
special  stain  for  the  demonstration  of  the  fibrils,  as  may  be  seen  by 
going  over  the  stains  for  this  purpose  in  LEE.  Teasing  has  been  of  no 
use  for  the  study  of  these  tissues. 

LITERATURE 

SPULER,  A.,  1897.     "  Beitrage  zur  Histologie  und  Histogenese  der  Bind-  und  Stutzsub- 
stance,"  Anal.  Hefle,  1897,  p.  115. 


'conn.  t.  c. 


THE    SUPPORTING  AND    CONNECTIVE  TISSUES:   SPECIALIZED 
TENSILE  FORMS 


It  is  right  to  decide  the  degree  of  special- 
ization of  a  tissue  by  the  degree  to  which  its 
characteristic  features  are  developed  either  quan- 
titatively or  qualitatively.  Thus  we  may  exam- 
ine, as  a  specimen  of  highly  specialized  tensile 
connective  tissue,  the  tendon  connecting  a  muscle 
with  a  bone  in  a  mammal  or  bird  (Fig.  65). 

The  first  impression  one  receives  on  looking 
at  a  longitudinal  section  of  tendon  is  a  great 
amount  of  fine,  strong,  parallel  strands  in  which 
no  nucleus  is  present.  Nuclei  can  be  seen, 
however,  between  the  fibrous  strands;  and  a 
closer  inspection,  especially  in  transverse  sec- 
tions, will  disclose  the  cytoplasm  belonging  to 
each  nucleus,  wedged  in  between  the  fibrils  and, 
like  them,  drawn  out  extensively  in  the  direction 
of  the  fibrils.  The  entire  surface  of  the  cyto-  Fl(J 
plasm  is  in  contact  with  some  fibrils.  In  turn  from 
each  fibril  throughout  its  length  is  in  contact 
with  the  cytoplasm  of  some  cell. 


of  tendon 
cow.   conn.t.c.,  con- 

nectiv*-tissue  cells  seen  from 
the  side  and,  m  one  case,  from 

the  surface. 


64 


HISTOLOGY 


It  will  now  be  realized  that  the  fibrils  are  not  cellular  elements, 
but  extra-cellular  structures  belonging  to  and  elaborated  by  the  cells 
we  have  found  lying  among  them.  The  cells  present  several  odd  and 
extreme  features  by  which  they  differ  from  a  typical  cell.  The 
nucleus  is  not  spherical,  but  drawn  out  so  that  it  appears  in  our 
specimen  like  a  bit  of  red  thread  and  needs  a  high  power  to  show 
its  nuclear  organs.  Again,  the  cytoplasm  does  not  lie  as  a  spherical 
body  around  the  nucleus,  but  is  drawn  out  from  it  in  three,  four,  or 
five  longitudinal  and  lateral  plate-like  projections,  which  have  been 
appropriately  called  "  wings,"  leaving  a  very  small  proportion  of  the 
cytoplasm  immediately  surrounding  the  nucleus.  It  also  reaches  for 
a  considerable  distance  beyond  the  two  ends  of  the  nucleus. 

The  nucleus  is  long  and  thin  because  it  must  lie  in  the  midst  of  fibrils 
which,  at  frequent  intervals,  are  under  enormous  strain.  If  spherical, 
the  nucleus  would  be  crushed  and  would  interfere  with  the  efficiency 
of  the  fibrils.  The  cytoplasm  is  drawn  out  in  several  directions  because 
it  must  be  in  contact  with^he  fibrils  to  support  them.  It  is  drawn  out  in 
the  wing-like  form  because  this  form  allows  its  duties  to  be  performed 
without  injury  to  itself  or  interference  with  the  work  of  the  fibrils.  It  is 
a  manifest  mechanical  truth  that  a  given  number  of  fibrils  all  pulling 
together  should  be  placed  parallel  to  each  other  and  side  by  side  for  the 

greatest  efficiency,  and  the  call  for 
efficiency  is  so  urgent  in  the  highly 
organized  tendon  that  the  cellular  ele- 
ments have  to  be  modified  to  give  the 
fibrils  every  opportunity. 

The  tendon  we  have  examined  is 
not  elastic  to  any  degree,  but  will 
break  before  it  will  stretch.  This 
condition  is  doubtless  obtained  by  its 
chemical  composition  and  demanded 
by  its  use.  There  are  other  connect- 
ive-tissue cells,  however,  which  form 
tendons  that  will  stretch. 

The  ligamentum  nuchae  of  an  ox 
or  other  mammal  shows  this  struc- 
ture (Fig.  66).  Here  the  cells  have  a 
different  structure  that  produces  an 
elastic  fibril  in  place  of  the  non-elas- 


.  t  c. 


FIG.  66. -Portion  of  ligamentum  nuchae     tjc    fibril.    This    elasticity   is    further 


of  ox.    conn.t.c.,  connective-tissue  cells. 


,         ,  ,     .  ,  11-1 

developed  by  the  net  work-  like  ar- 
rangements of  the  fibrils  which  are  woven  among  each  other  and  do 
not  lie  parallel.  The  fibrils  are  very  large  and  the  nuclei  of  the  cells 


HIGHER    TENSILE    CONNECTING    TISSUES 


are  flattened  out  somewhat,  but  not  drawn  out  as  in  the  tendon. 
These  cells  seem  to  have  formed  a  second  set  of  extremely  minute 
fibrils  which  lie  between  the  large,  heavy  fibrils  of  elastic  tissue  and 
at  points  appear  to  be  attached  to  them.  These  loose  fibrils  appear 
to  be  a  lax  connective  tissue. 

The  lower  forms  of  animals  show  but  few  examples  of  specialized 
ligaments,  owing  to  the  general  distribution  of  the  muscles  and  their 
large  areas  of  attachment.  Also  many  of  these  forms  have  no  skeleton, 
and  consequently  the  muscles  work  in  masses  of  connective  tissue  and 
in  a  series  of  opposed  planes. 

The  Crustacea  have  a 
rigid  exoskeleton  or  shell  and 
a  series  of  powerful,  quick- 
acting  muscles  that  require 
firm  attachment  to  the  inside 
of  the  shell.  They  are  at- 
tached by  very  short  liga- 
ments, as  is  described  below. 
No  long  ligaments  exist,  and 
therefore  when  a  muscle  does 
not  reach,  by  some  little  dis- 
tance, to  its  point  of  attach- 
ment, an  inwardly  produced 
process  of  that  point,  com- 
posed of  hypodermis  and 
cuticle,  reaches  in  to  meet 
the  muscle.  These  integu- 
mental  "ligaments"  are  char- 
acteristic of  the  Crustacea 
and  may  be  seen  by  exam- 
ining the  muscle  of  a  lob- 
ster's great  claw. 

A  tissue,  however,  that 
acts  as  a  very  short  ligament 
does  exist  in  the  lobster 
wherever  a  muscle  is  attached 
to  the  shell  or  to  one  of  its 
inside  processes.  It  is  not 

.          .  ,  ,  FIG.  67. —  Portion   of  the  new  integument  of  a  lob- 

COnnectlVC  tiSSUe  alone,  how-  ster>  Homarus.  Conn.t.n.,  connective-tissue  nucleus; 
ever,  that  Serves  this  pur-  bl.c.,  blood  cells;  mus.c.,  muscle  cells. 

pose.    The  simple,  columnar 

epithelium  cells  that  cover  the  outside  of  the  body  under  the  shell, 

which  they  form,  assume  part  of  this  duty   and  acquire  strength  to 


mus.'c. 


p*  $&  y 

P®    w.'». 


66  HISTOLOGY 

perform  it  by  the  development  of  strong  fibrils  in  their  cyto- 
plasm. 

Figure  67  shows  a  bit  of  shell  resting  on  the  columnar  epithelium 
that  formed  it.  In  the  right-hand  portion  of  the  figure  is  seen  this  epithe- 
lium in  an  undifferentiated  state,  with  its  base  resting  on  a  delicate  layer 
of  connective  tissue.  The  exact  boundary  between  the  base  of  the  epithe- 
lium and  this  connective  tissue  is  difficult  to  detect.  In  the  left-hand  part 
of  the  figure  can  be  seen  the  same  structures,  but  somewhat  modified, 
owing  to  the  fact  that  some  muscle  fibers  are  attached  to  them  at  this 
point. 

Both  the  connective-tissue  cells  and  the  epithelial  cells  have  modi- 
fied their  structure  and  united  to  act  as  a  short  ligament.  The  fibrils 
which  were  weakly  developed  in  the  undifferentiated  epithelial  cell 
have  become  heavy  and  strong,  and  lie  in  parallel  positions  in  this  differ- 
entiated one.  The  weak  fibrils  that  run  in  irregular  courses  and  partly 
parallel  to  the  shell  in  the  first  set  of  connective-tissue  cells  have  become 
stronger  and  assumed  a  position  of  greatest  efficiency,  at  right  angles 
to  the  shell  and  in  line  with  both  muscle  and  epithelial  fibrils,  in  the  tissue 
seen  in  the  second  group.  As  in  most  connective-tissue  cells,  the  actual 
boundaries  are  too  complicated  and  weakly  marked  to  be  seen. 

These  three  sets  of  fibrils,  the  epithelial  cell  fibrils,  the  connective- 
tissue  cell  fibrils  and  the  muscle  fibrils,  have  been  joined  individually 
into  a  set  of  long  and  apparently  continuous  fibrils,  of  which  the  first 
two  portions  form  the  passive  ligamentous  part,  while  the  last  acts  as 
the  actively  contractile  portion.  The  strain  thus  comes,  in  all  three 
of  these  cells,  not  on  the  cytoplasm  of  the  cells  but  on  special  cell-organs 
which  have  been  formed  by  the  cytoplasm  to  meet  it. 

This  mode  of  muscle  attachment  is  found  throughout  the  lobster's 
body.  In  some  places  the  connective-tissue  elements  are  so  small  as  to 
be  apparently  absent,  and  it  would  seem  possible  that  in  some  attach- 
ments they  were  absent  altogether  and  the  muscle  was  joined  directly 
with  the  epithelium.  This  condition  is  clearly  true  in  some  lower  Crus- 
tacea, and  has  been  figured  by  Schneider  in  Branchipus. 

Study  in  this  connection  :  the  mode  of  attachment  of  the  closing  muscle 

to  the  shell  in  a  Unio;  the  attachment  of  an  Anomia  to  a  rock  and  the 

attachment  of  muscles  to  the  head-cartilages  in  the  squid  or  Octopus. 

Technic.  —  Use  the  same  methods  that  were  used  for  the  simpler 

tensile  forms. 

LITERATURE 
Read  the  same  articles  that  were  suggested  for  the  preceding  parts. 


HIGHER  RIGID  SUPPORTING   TISSUES 


67 


THE   SUPPORTING  CONNECTIVE  TISSUES:   SPECIALIZED   RIGID 

FORMS 

We  shall  consider  as  specialized  forms  of  rigid  connective  tissues,  all 
those  cases  in  which  a  large  number  of  cells  work  collectively  to  produce 
a  single  supporting  structure  or  homogeneous  supporting  material  for 
the  body.  Such  materials  are  usually  produced  inside  of  the  body  by 
mesodermal  tissues  and  form  an  endoskeleton.  As  examples  of  such, 
we  shall  study  the  tissues  that  form  the  skeletal  structure  of  the  common 
sponge,  others  that  build  up  the  cartilages  of  the  squid  and  the  verte- 
brate animals,  and  those  that  form  the  bone  of  the  mammals.  Many 
integumental  structures  are,  in  effect,  skeletal.  We  therefore  refer  the 
student  to  Chapter  XX,  Part  A,  to  read,  in  this  connection,  the  descrip- 
tions of  the  shell  of  an  arthropod  and  the  scale  of  a  teleost  fish.  The 
"pen"  or  shell  of  the  cephalopod  mollusk,  while  an  ectodermal  product, 
functions  purely  as  an  internal  organ 
of  support  and  will  be  treated  of  in 
this  chapter. 

The  Cellulose  Skeleton  of  Euspon- 
gia  (Fig.  68).  —  In  this  sponge  the 
skeleton  consists  of  long,  curved,  and 
branching  rods  of  a  material  that  re- 
sembles cellulose  and  is  variously 
branched  and  arranged  to  loosely 
support  and  uphold  the  soft  and  deli- 
cate tissues  of  the  body. 

The  particular  cells  that  are  respon- 
sible for  the  production  of  this  fiber 
are  part  of  the  middle  layer  of  con- 
nective tissue,  and  are  indistinguish- 
able from  their  fellows  until  a  fiber  is 
to  be  made  in  their  neighborhood. 

Then  they  accumulate  in  the  path 
of  the  future  fiber  and  its  branches, 
and  secrete  and  pour  out  the  material 
of  which  it  is  formed.  While  the  fiber  FIG  ^'-p^rf'  a  newly  forming  skeletal 

is  growing,  they  remain  attached  tO  it,        fiber  of  Euspongia.     (From  SCHNEIDER 

forming  at  this  time  an  epithelium-like      after  F-  E-  SCHULTZE-> 
layer  of  cells  around  it.    They  differ  in  shape  and  other  features  from 
other  mesoglcea  cells  during  this  period  of  fiber  production,  but  when 
it  is  finished  they  retire  and  again  assume  the  form  of  the  other  con- 
nectivet-issue  cells. 


68 


HISTOLOGY 


Shell  of  Loligo  Pealii. — A  characteristic  organ  of  the  mollusca  is 
the  shell,  a  thick  layer  of  chitin,  usually  reenforced  with  carbonate  of 
lime,  that  is  developed  on  all,  or  a  part,  of  the  mantle. 


-mus. 


FlG.  69.  — Transverse  section  through  quill  (shell)  of  a  squid,  Loligo.     qu.,  quill;  sec.ep.,  secret- 
ing epithelium  of  quill;  mus.,  muscle  of  mantle  wall;  car.,  cartilage. 

In  the  squid  this  "quill"  is  carried  inside  the  body  by  an  invagination 
of  the  surface  on  which  it  lies  (Fig.  69).  The  formative  epithelial  cells, 
here  a  simple  columnar  epithelium,  surround  this  shell,  which  is  of  chitin 
alone  without  any  lime,  and  build  it  up  from  all  sides.  Its  beautiful 
ribbed  structure  makes  it  an  ideal  organ 
of  support.  It  is  laid  down  in  layers, 
as  are  all  mollusk  shells.  At  the  points 
where  it  is  thickest  the  cells  that  form 
it  are  also  longest,  and  form,  in  conse- 
quence, the. thickest  epithelium. 

Another  kind  of  rigid  connective  sub- 
stance is  found  in  the  interior  of  the 
body  in  many  animals  which  is  a  mass 
of  material  formed  by  many  cells  work- 
ing jointly.  The  tissue  is  called  cartilage, 
and  the  substance  formed  by  its  cells 
is  composed  of  several  organic  materials. 

The  cartilage  found  in  the  cephalopod  mollusks  shows  all  sides  of 
the  development  and  structure  of  this  tissue,  and  will  represent  the  car- 
tilages in  general  (Fig.  70).  It  begins  as  a  loose  collection  of  mesoderm 


FIG.  70.  —  Developing  cartilage  of  a 
young  squid.  Mitotic  figures  very 
abundant.  X  1200. 


HIGHER  RIGID  SUPPORTING    TISSUES  69 

cells  with  large  nuclei  and  small  cytoplasmic  bodies  that  are  fairly  com- 
pact but  yet  send  out  numerous  branches  which  anastomose  with  one 
another.  The  deposition  of  the  cartilage  substance,  called  the  matrix, 
begins  at  the  principal  boundary  of  the  cytoplasmic  body,  and  the  ma- 
terial first  appears  as  a  thick  rim  of  a  hyaline  substance  surrounding 
each  cell.  These  rims  coalesce  with  each  other  and  new  concentric 
layers  are  laid  down,  always  next  to  the  cell  and  by  the  agency  of  its 
cytoplasm.  The  material  of  the  matrix  changes  in  chemical  reaction 
as  it  grows  older,  and  in  some  forms  of  cartilage  the  concentric  layers  of 
which  it  is  composed  are  plainly  visible.  Usually  they  are  not  so  unless 
by  special  treatment  under  the  microscope. 

In  most  cartilages,  and  especially  in  that  of  the  squid,  which  we  are 
studying,  the  layers  of  matrix  do  not  entirely  shut  in  the  cell.  The  pro- 
cesses of  the  cytoplasm  pass  through  them  and  unite  with  the  similar  pro- 
cesses from  other  cells,  thus  forming  pathways  from  cell  to  cell  through 
the  matrix,  for  the  passage  of  food  and  other  materials  which  are  carried 
by  the  protoplasmic  processes. 

As  the  cartilage  grows,  its  cells  multiply  in  order  that  there  may 
always  be  enough 
cells  scattered 
through  the  matrix 
to  have  full  control 
of  its  growth,  sup- 
port, or  atrophy  at 
all  times.  This 
multiplication  is  by 
mitosis,  although 
amitosis  possibly 
occurs  in  later 

growth        processes      FIG.  71.  — Cartilage  of  an  adult  squid.     Intercellular  canals  partly 

(Fig.  71),  and  when  shown,    x  650. 

the    two    daughter 

cells  are  completed,  they   continue   the   formation  of  the  matrix  as 

before,  from  every  part  of  the  surface  of  their  cell  bodies. 

This  results  in  the  formation  of  a  thin  but  growing  plate  of  the  matrix 
between  them,  which  is  continued.  As  several  divisions  usually  occur 
in  succession,  the  cells  are  often  found  arranged  in  groups  of  two,  four, 
or  eight,  as  in  Figure  71.  The  cartilage  cells  store  glycogen,  and  when 
prepared  for  the  microscope  are  usually  shrunken  by  the  loss  of  this 
glycogen,  which  readily  dissolves  in  the  fluids  used  to  harden  the  tissue. 
This  simple  cartilage  is  one  of  a  large  group  of  tissues  of  which  there 
are  several  kinds. 

The  simplest  kind  is  that  just  described,  and  is  known  as  a  hyaline 


HISTOLOGY 


cartilage.     A  variety  in  which  elastic  fibrils  have  been  developed  in 

the  matrix  is  known  as  elastic  cartilage ;  and  still  another,  in  which  white 

&.-..-  .•.•<.-:-•«.--<.-:  .y--^-....  tendon   fibrils   have   pene- 

^^ '-^4i*>j :-0  :-^^  :^:''\9^i^^.  trated  the  hyaline   matrix, 

•*-i;vx ;'.-'X:*iir, /.-'...-; ::  •  .y  .x^jp^;  is  known  as  fibrous  carti- 

M®J^iS:SSsto0  lage. 

The  bone  of  the  verte- 
brates is  perhaps  the  most 
highly  developed  and  effi- 
cient of  the  rigid  connective 
tissues.  It  is  formed  from 
the  mesodermic  tissues  and 
is  always  an  internal  struc- 
ture. It  has  many  varieties 
in  the  groups  of  chord  ate 
animals,  and  we  shall  select 
the  best-known  form  as  an 
example,  the  bone  of  the 
mammals  and  of  man. 

This  structure  is  formed 
in  two  ways,  producing  two 
slightly  different  structures. 
The  most  primitive,  per- 
haps, is  the  endochondral 
bone,  formed  by  the  trans- 
formation of  hyaline  carti- 
lage (Fig.  72).  The  second 
is  formed  directly  from  the 
connective  tissue,  especially 
that  which  surrounds  a  car- 
tilage or  a  bone.  This  is 
known  as  perichondral  bone 
formation.  It  can  be 
formed  in  the  connective 
tissue  quite  apart  from  any 
cartilage.  Endochondral 
bone  is  formed  as  follows 
in  an  embryonic  finger  car- 
tilage. 

The  bone  transformation  begins  in  the  middle  of  the  joint  and  ad- 
vances as  a  curved  line  in  both  directions,  distad  and  proximad.  This 
line  is  the  point  at  which  the  cartilage  may  be  said  to  change  into  bone 
and,  as  it  moves,  the  cartilage  cells  in  front  of  it  go  through  a  series 


FIG.  72.  —  Reconstruction  of  cartilage  into  bone,  car.c., 
cartilage  cells  in  successive  stages  of  degeneration ; 
0s/.,  osteoblasts;  gi.c.,  giant  cells  or  osteoclasts;  b., 
young  bone ;  bl.c.,  blood  cells. 


HIGHER  RIGID   SUPPORTING    TISSUES  J I 

of  changes,  beginning  when  it  has  arrived  within  a  certain  distance 
of  them.  Also  when  it  has  passed  it  does  not  leave  fully  formed 
bone  behind  it.  Only  when  it  has  passed  a  certain  distance  does  the 
tissue  finally  appear  completed.  The  process  thus  occupies  a  zone  of 
some  width.  This  movement  is  called  the  line  or  wave  of  ossification. 
At  the  point  where  it  begins  a  blood  vessel  loop  enters  the  future  bone, 
bringing  with  it  the  various  bone-making  cells  of  the  perichondrium 
and  a  blood  supply  for  circulation. 

As  the  line  of  ossification  approaches  within  a  certain  distance  of 
any  point  of  the  cartilage,  the  cartilage  cells  occupying  that  point  begin 
to  change  in  form  as  well  as  position.  The  change  in  position  is  most 
marked  (see  Fig.  72).  Instead  of  lying  scattered  at  random  with 
only  a  somewhat  regular  proximity  of  the  daughter  cells  of  common 
ancestry,  the  cartilage  cells  arrange  themselves  in  more  or  less  regular 
rows  placed  at  right  angles  to  the  line  of  advancing  ossification. 

How  the  cells  move  in  order  to  place  themselves  thus  is  not  known. 
As  the  spaces  that  they  occupy  in  the  matrix  become  wider  and  narrower 
as  well  as  larger  than  before,  and  as  the  increasing  size  of  this  space  shows 
that  the  cartilage  cells  eat  away  the  matrix,  it  is  probable  that  they  move 
by  dissolving  the  matrix  before  them.  Once  in  line,  the  dissolution  of 
matrix  goes  rapidly  on,  and,  as  the  cell  spaces  are  nearer  to  those  next 
them  in  the  line  than  to  those  in  the  neighboring  lines,  it  follows  that 
the  spaces  in  each  line  break  through  their  barriers  and  form  channels 
with  irregular  strands  of  matrix  lying  between  them.  Sometimes  two 
or  more  channels  will  unite  to  become  one  larger  one.  The  cartilage 
cells,  meanwhile,  slowly  swell  up,  lose  their  staining  power,  and  by 
the  time  the  spaces  are  broken  into,  have  entirely  disintegrated. 

When  the  process  first  began  at  the  center,  a  "bud"  of  the  perichon- 
dral  membrane,  a  connective  tissue  covering  the  cartilage,  was  evagi- 
nated  into  the  space  left  by  the  first  dissolution  of  matrix,  carrying  with 
it  a  loop  of  blood  vessel  and  its  own  inner  layer  of  modified  cells,  that 
have  the  power  of  secreting  various  salts  of  lime  and  depositing  them  on 
whatever  surface  they  may  rest  against. 

A  layer  of  these  cells  is  pushed  up  into  each  of  the  channels  and 
against  the  surface  of  its  walls,  the  remnants  of  the  cartilage  matrix. 
Here  they  begin  the  deposition  of  lime  salts,  the  materials  of  which  are 
supplied  them  by  the  loop  of  blood  vessel  that  grows  into  each  channel. 
These  vessels  follow  closely  behind  the  perichondral  cells  which,  in  their 
new  position  and  function,  are  called  the  osteoblasts  or  bone-making  cells. 

The  osteoblasts  lay  down  the  lime  salts,  first  in  the  remnant  of  car- 
tilage matrix  and  then  in  successive  internal  layers.  Between  each  neigh- 
boring pair  of  layers  some  of  the  osteoblasts  are  left  in  small  chambers 
of  the  bone  called  lacuna.  In  this  position  they  are  called  bone  cells. 


HISTOLOGY 


FIG.  73. —  Transverse  section  of  a  Harversi an  sys- 
tem of  bone,  h.c.,  Harversian  canal;  can.,  can- 
aliculi;  lac.,  lacunae;  la.,  lam  ell  ae. 


Provision  is  also  made,  by  fine  channels  called  canaliculi,  for  the 
\  ffJtf/ht  processes  of  the  bone  cells  to 

Al        Jfil/MMk  reach  and  anastomose  with  each 

other,  thus  forming  radial  path- 
ways for  the  transportation  of 
food  and  other  matter  from  the 
central  canal,  or  Harversian 
canal  as  it  is  now  called,  to  the 
entire  group  of  bone  cells  be- 
tween the  surrounding  lamellae 
of  bone.  The  whole  structure, 
central  canal  containing  blood, 
nerve  and  reserve  bone  cells, 
together  with  the  surrounding 
lamellae  and  the  bone  cells 
between  them,  is  called  an 
Harversian  system  (Fig.  73). 

Bone  is  made  up  of  a  mass 
of  these  structures  formed  not 
only  by  the  transformation  or,  to  be  more  accurate,  the  reconstruc- 
tion of  cartilage,  but  also  by 
the  activity  of  the  membrane 
surrounding  the  bone,  the  peri- 
osteum, which  is  the  same  as  the 
perichondrium.  This  periosteum 
is  made  up  of  several  connective- 
tissue  layers,  the  inner  of  which 
are  modified  into  compact  cells 
of  some  size,  the  osteoblasts, 
which  are  the  same  as  those  that 
entered  into  the  cartilage  to  re- 
place its  matrix  with  the  bone  08t— ->  -^ 
substance. 

Where  this  membrane  is  in 
contact  with  the  surface  of  a 
bone,  such  as  the  one  whose 
reconstruction  from  cartilage  we 

have  jUSt  described,  its  inner  layer  FlG-  74-  -Formation  of  bone  by  the  periosteum. 

.  ost.,  osteoblasts  laying  down  bone  and  becom- 

Of  OSteoblastS  begin   tO    lay  down  ing  inclosed  in  this  bone  as  bone  cells;  b,  bone 

bone    material    (Fig.    74).     They  substance.     (Part  of  a  figure  by  LEWIS  from 

\     &  .  '    ,      ,         J  STOHR'S  Histology.) 

do   not    do   so  in  simple  layers 

but  in  long,  hollow  grooves  whose  edges  are  built  up  more  rapidly  than 

the  rest,  finally  meeting  and  inclosing  a  part  of  the  osteoblasts  and  a 


FAT  73 

core  of  connective  tissue  containing  a  blood  supply.  This  core  then 
operates  with  its  peripheral  osteoblasts  to  form  an  Harversian  system, 
just  as  the  osteoblasts  did  in  the  cartilage  channels  during  endochondral 
bone  formation.  The  rising  edges  of  the  future  Harversian  systems 
are  laid  down  in  a  connective-tissue  basis  that  precedes  them.  This 
is  shown  in  Figure  74. 

One  other  histological  process  in  connection  with  bone  formation 
should  be  studied  here.  As  most  bones  increase  in  diameter,  they  become 
hollow  in  the  center,  this  space  being  filled  with  the  marrow.  In  order 
that  this  may  occur,  some  of  the  bone  material  must  be  removed,  and  this 
occurs  as  follows. 

Large,  heavy  cells  appear  in  the  Harversian  canals  and,  applying 
their  bodies  to  the  bone  material,  they  proceed  to  absorb  it  and  remove 
it.  They  probably  do  this  by  secreting  some  acid  in  the  cytoplasm  and 
dissolving  the  salts,  which  are  then  carried  away  by  the  blood.  It  would 
be  interesting  to  know  more  about  the  acids  that  are  used  and  what 
becomes  of  the  salts  in  solution;  they  may  be  used  over  again  by  the 
osteoblasts.  These  large  cells  that  remove  the  bone  substance  are 
called  the  giant  bone  cells  or-osteoclasts  (see  Fig.  72,  gi.c.).  They 
usually  contain  several  nuclei. 

Technic. — The  technic  of  this  group  of  tissues  is. somewhat  diffi- 
cult on  account  of  the  hardness  of  some  of  them.  To  see  bone  properly, 
one  should  study  both  decalcified  and  ground  sections  of  the  tissue. 
Developing  bone  is  easily  cut  in  both  paraffin  and  in  celloidin  when  the 
fixing  fluid  has  been  an  acid  one  like  Flemming's  fluid  and  most  of  the 
others.  Any  of  these  tissues  may  be  first  fixed  and  subsequently  decal- 
cified with  nitric  acid  in  connection  with  phloroglucin  (see  Lee).  Some 
of  the  shells  must  be  ground,  and  then  the  soft  parts  are  nearly  always 
destroyed.  Many  of  the  tougher  integuments  may  be  cut,  at  some 
expense  in  the  sharpening  of  knives. 

LITERATURE 

Read  the  general  literature  on  the  simple  rigid  forms  and  then  see  the  descriptions  of 
bone  and  its  histogenesis  in  some  good  medical  histology. 


FAT 

Various  kinds  of  cells  can  store  up  prepared  food  materials  in  their 
cytoplasm  as  a  reserve  supply  in  the  economy  of  the  animal.  Chief 
among  these  are  the  otherwise  unspecialized  connective-tissue  cells  of 
the  vertebrates  that  are  able  to  take  such  materials  into  the  cytoplasm 
in  the  form  of  various  fatty  acids.  These  are  known  as  the  fat  cells,  and 


74 


HISTOLOGY 


uoies.   (After  LEWIS.) 


they  are  found  among  practically  all  of  the  mesodermal  parts  of  the 

body,  acting  also  in  some  degree  as  buffers  and  fillers  for  otherwise 
unoccupied  spaces.  Their  content  is  often 
called  upon  for  food  when  anything  goes 
wrong  with  the  other  means  of  nourish- 
ment, and  they  give  up  this  substance 
readily  to  the  blood  to  be  carried  where 
it  is  needed. 

The  fat  of  the  mammal  may  be  best 
studied  by  tracing  its  genesis  in  the  em- 
bryo (Fig.  75).  We  shall  do  this  in  the 
human  embryo,  beginning  with  sections  of 
the  skin  from  a  foetus  of  between  four  and 
five  months'  development.  These  cells  are 
at  to  exactly  the  same  as  the  other  con- 
nective-tissue  cells  lying  around  them. 
When  they  begin  to  differentiate,  a  num- 

ber of  tiny  droplets  of  the  fatty  substance  appear  in  their  bodies, 

and  as  these  droplets  grow  in  size  they  push  the  nucleus  to  one  side. 

In   growing,   they    merge    together 

until  they  form  but  one  large  drop. 

For  some  time  after  the  large  drops 

of  fat  are  formed,  other  small  and 

new  drops  arise  in  the  cytoplasm, 

and  later  join  the  large  single  drop, 

until,  at  the  maturity  of  the  drop,  it 

is  many  times  the  size  of  the  origi- 

nal cell  and  shows  the  cytoplasm  of 

this  cell  lying  around  it  as  a  thin 

cover,  thickened  enough  at  one  side 

to  contain   the   nucleus.      Enough 

of  the  undifferentiated  connective- 

tissue   cells    remain    in    the    tissue 

to    hold    it    more    or    less    firmly 

together.     Figure  76  is  drawn  from 

the  integumental  fat  of  the  chicken 

in  which   these  relations  are  par- 

ticularly    favorable     for     observa- 

tion. 

There  are  many  cells  in  the  in- 

vertebrate forms  that  store  up  pre- 

pared food  materials.     In  no  case,  however,  is  this  material  exactly  like 

the  vertebrate  fat  in  chemical  composition.     In  some  cases  it  more  nearly 


FIG.  76.  —  Fat  cells  lying  in  connective-tissue 
reticulum.  Skin  of  chicken,  nu.,  fat  cell 
nuclei.  X  870. 


FAT 


75 


resembles  the  materials  stored  in  the  vertebrate  liver.  The  cells,  how- 
ever, seem  to  store  them  as  reserve  food  supplies,  as  is  demonstrated  by 
the  fact  that  before  par- 
ticular growth  periods, 
when  the  creature  will  be 
unable  to  eat,  huge  quan- 
tities of  these  materials  are 
collected  in  the  cells,  and 
when  the  growth  and  molt- 
ing is  over  they  have  been 
used. 

The  " fat-bodies"  of  the 
insects  furnish  an  example. 
These  bodies  are  strings  of 
mesodermal  cells  lying  in 
the  body  cavity  of  various 
larvas.  Figure  77  shows  a 
section  of  such  a  fat-body 
from  the  larva  of  a  Regalis 
moth.  This  larva  had  just 
completed  a  molt,  and  the  cells  do  not  therefore  show  as  much  food 
material  as  they  would  if  the  tissue  had  been  taken  just  before  the 
molt. 

The  Crustacea  also  present  analogous  tissues  that  are  probably  used 
as  stores  of  reserve  foodstuffs,  especially  just  before  the  molting  period. 

Technic.  — The  fats  may  be  cut  like  any  other  tissues,  and  such  sections 
yield  very  good  results,  especially  when  one  is  already  well  acquainted 
with  the  tissues.  To  get  so  acquainted  as  well  as  to  decide  with  accuracy 
if  a  given  vertebrate  tissue  contains  fat  or  not,  the  material  should  be 
cut  fresh  with  a  freezing  microtome  and  the  sections  stained  with  some 
of  the  specific  fat  stains,  of  which  there  are  a  number  in  the  list  of  ana- 
line  dyes.  Osmic  acid  will  also  stain  the  fat  substance  a  good  black  that 
can  be  readily  recognized. 


FIG.  77.  —Cells  from  fat  body  of  the  insect,  Regalis. 
X  580. 


LITERATURE 

KOSCHEVNIKOFF,  G.  A.,  1900.    "Uber  den  Fettkorper  und  die  Oenocyten  der  Honigbiene 

(Apis  mellifica),"  Zool.  Anz.,  Band  XXIII. 
WIELOWIEJSKI.  H.  v.,  1883.    "  Uber  den  Fettkorper  von  Corethra  plumicornis  und  seine 

Entwicklung,"  Zool.  Anz.,  1883.  VI  Jahrgang,  pages  318-322 
Read  of  the  development   and   structure  of  human  fat  in  any  of  the  best  medical  his 

tologies. 


CHAPTER   VIII 
THE   TISSUES    OF    MOTION 

MOTION  is  an  almost  universal  attribute  of  protoplasm.  Practically 
all  cells  can  move  some  part  of  their  body,  even  if  they  have  no  means 
of  moving  the  body  as  a  whole  from  place  to  place.  Thus,  the  motion 
may  consist  of  internal  operations,  as  circulation,  ciliary  movement,  or 
the  many  and  varied  acts  of  cell  division. 

Or  it  may  be  exhibited  as  the  contraction  and  relaxation  of  the  cell- 
body  as  a  whole,  as  in  some  epithelial  and  other  cells.  Usually  the  act 
of  relaxation,  as  performed  by  elastic  and  other  non-living  parts  of  the 
cell,  is  supplanted  by  another  motion  in  a  different  plane,  which  acts  as 
a  counter  to  the  first  movement,  restoring  the  mass  to  its  previous  form 
and  shape. 

In  some  free  cells,  as  Amoeba  and  the  white  blood  corpuscles,  these 
movements  result  in  complex  flowing  and  creeping  motions  that  move 
the  entire  cell  from  place  to  place.  In  other  unicellular  forms  the  more 
or  less  numerous  cilia  or  flagella  on  the  body,  which  is  rigid,  move  the 
creature  in  any  direction  and  at  high  rates  of  speed.  Both  of  these  forms 
of  movement  are  found  in  the  simply  organized  and  small  unicellular 
animals,  but  in  the  great  majority  of  higher  form  this  function  of  moving 
is  all  performed  by  special  cells,  only,  of  the  organism,  and  these  cells 
present  as  high  and  complete  a  specialization  as  we  find  in  nature.  We 
shall  call  them  the  muscle  cells.  The  cytoplasm  of  muscle  cells  is  of  char- 
acteristic granular  structure  and  is  called  sarcoplasm.  The  granules 
are  known  as  myochondria. 

The  fully  specialized  muscle  cell  can  contract  with  a  force,  rapidity, 
and  quick  succession  far  beyond  the  power  of  less  specialized  protoplasm. 
It  gets  this  greater  power  and  efficiency  from  the  development  in  its 
cytoplasm  of  certain  long  thread-like  regions  of  contractile  substance, 
the  muscle  fibrils.  These  fibrils  are  the  cell-organs  of  contraction  and 
vary  in  appearance  and  number  according  to  the  needs  of  the  tissue. 
They  are  formed  as  plastid-areas  of  the  sarcoplasm. 

They  lie  parallel  to  the  long  axis  of  the  cell  and  in  the  direction  of 
motion.  They  vary  in  the  completeness  with  which  they  are  differen- 
tiated out  of  the  sarcoplasm  of  the  muscle  cell.  They  also  differ  in  struc- 
ture so  that  they  may  be  classed  as  striated  or  non-straited  fibrils.  This 

76 


THE    TISSUES   OF  MOTION 


77 


FIG.  78. — Transverse  section  of  a  striated  muscle 
fiber  from  the  bat's  tongue.  To  show  the  com- 
paratively great  amount  of  sarcoplasm.  x  i 200. 


striation  or  segmentation  consists  of  a  transverse  division  of  the  body 
of  the  fibril  into  a  larger  or  smaller  series  of  equal  w.orking  units  called 
muscle  elements,  sarcous  elements,  or  sarcomeres,  and  these  have  a  defi- 
nite structure  which  is  exactly  the  same  in  all  the  fibrils  in  a  given  cell. 
Each  sarcous  element  itself  has  a  definite  and  symmetrical  segmen- 
tation which  varies  in  different  forms,  and  changes  during  the  con- 
traction and  expansion  of  any 
given  example.  This  shall  be 
described  in  detail  further  on. 

The  non-striated  fibrils  are 
perfectly  smooth.  In  some 
mollusks  a  peculiar  woven  ap- 
pearance of  the  fibers  is  prob- 
ably due  to  the  irregular 
arrangement  of  its  weakly  stri- 
ated fibrils. 

Some  muscle  cells  may  con- 
tain but  few  of  the  fibrils 
scattered  singly  through  the 
sarcoplasm.  Again,  in  others, 
the  fibrils  are  more  numerous 
and  are  gathered  together  into 

a  number  of  column-like  (see  Fig.  92,  lobster's  cardiac  muscle)  or 
plate-like  groups  (see  Fig.  82).  These  groups  will  be  spoken  of  as  fibril- 
bundles.  When  cut  in  transverse  sections  they  are  known,  in  mamma- 
lian muscle,  as  Conheim's  fields.  In  the  bat  the  fibrils  are  all 
separated  and  are  not  so  grouped  (Fig.  78).  Sometimes  the  fibrillae 
are  so  numerous  and  form  such  a  homogeneous  mass  that  they  fill  the 
greater  part  of  the  cell,  only  leaving  room  for  the  nucleus  and  a  single 
cone-shaped  area  of  sarcoplasm  at  each  end  of  it. 

Each  muscle  fibril  is  usually  surrounded  by  a  differentiated  zone  of 
sarcoplasm,  which  may  be  called  the  cement  substance.  This  cement 
substance  is  common  to  all  the  fibrils  in  a  bundle,  in  some  forms  of 
muscle  (see  Fig.  82).  In  others  it  is  not,  and  in  some  it  is  not  apparent 
at  all. 

Any  conceptions  of  the  cause  of  the  mechanism  of  motion  that  we 
may  entertain  must  center  around  the  fibril.  Have  we  here  an  inten- 
sification of  the  same  phenomena  that  occur  in  an  Amceba  when  it  con- 
tracts, or  was  the  first  fibril  a  new  organ  that  enabled  its  possessor  to  do 
more  than  the  life  forms  had  been  able  to  do  up  to  that  time  ? 

Engelmann  has  given  us,  so  far,  the  best  theoretical  explanation  of 
the  motion  of  the  fibrillated  muscle  cell.  In  this  explanation  the  fibril 
is  supposed  not  to  have  any  "vital"  power  of  moving,  but  to  be  an  inert 


78  HISTOLOGY 

secretion  of  the  cell-  and  a  substance  particularly  sensitive  to  several 
chemical  or  physical  laws  which  rule  that,  when  warmer,  the  substance 
must  absorb  water,  and  that  when  such  a  substance  absorbs  water  it 
must  become  thicker  and  shorter. 

Then,  assuming  that  the  plasma  of  the  muscle  cell,  which  surrounds 
all  the  fibrils  in  it,  is  filled  with  some  secretion  substance  that  gives  heat 
when  combined  with  oxygen,  and,  assuming  that  the  motor  nerve  impulse 
causes  oxygen  to  unite  with  this  substance,  we  can  imagine  the  sudden 
warming  of  the  watery  plasma  by  the  rapid  oxidation  of  the  heat  secre- 
tion and  the  sudden  shortening  of  the  fibrils  by  the  absorption  of  water. 

This  explanation  does  away  with  all  unknown  vital  factors,  except 
a  secretory  activity  of  protoplasm  that  enables  it  to  produce  the  easily 
oxidizable  material,  and  a  nerve  stimulus  that  can  cause  oxygen  to  sud- 
denly unite  with  this  substance.  The  secretion  power  and  the  nerve 
stimulus  remain  to  be  explained. 

Another  but  less  satisfactory  explanation  of  more  remote  causes  is 
Schaefer's  theory  whereby  the  isotropic  or  light  substance  is  supposed 
to  retire  into  a  series  of  longitudinal  channels  in  the  anisotropic  sub- 
stance, thus  swelling  and  shortening  the  latter.  This  process  involves 
electrical  and  chemical  changes.  It  is  not  as  clear  as  the  first. 

The  cell-membrane  which  surrounds  a  muscle  cell  is  known  as  the 
sarcolemma.  In  a  large  number  of  forms  this  membrane  is  reenforced 
by  the  closely  applied  bodies  of  connective-tissue  cells,  in  which  case 
there  has  been  no  change  of  name  and  the  entire  covering  is  still  called 
the  sarcolemma.  In  most  cases  it  is  practically  impossible  to  demon- 
strate a  sarcolemma  apart  from  this  connective  tissue. 

A  muscle  cell  is  always  in  bodily  connection  with  a  nerve  cell  which 
controls  it.  The  organ  by  which  this  contact  is  effected  is  known  as  a 
motor  nerve  ending. 

The  muscle  cells  are  of  several  shapes  (Fig.  79).  Spindle  shape  and 
tapering  to  pointed  ends  (D),  elongated  cylinders  with  blunt  ends  (B\ 
cubical  cells  arranged  end  to  end  (A)  are  the  shapes  usually  found, 
while  epithelial  cells  with  the  base  flattened  out  and  converted  into 
muscle  (C)  and  branching  cells  are  some  of  the  unusual  forms.  The 
spindle-shaped,  cubical,  and  epithelial  cells  are  usually  mononuclear, 
while  the  cylindrical  are  nearly  always  multinuclear  cells  or  syncytia. 

In  many  small  and  simply  organized  animals  the  muscle  cells  are 
placed  apparently  at  random  through  the  tissues  of  the  body  or  in  layers 
on  its  surface.  At  the  point  where  the  animal  attains  to  developed 
powers  of  movement,  however,  these  scattered  fibers  become  localized 
into  groups  which  are  placed  in  positions  where  their  force  can  be  exerted 
to  the  best  advantage.  Such  groups  are  called  the  muscles.  They  can 
operate  to  move  the  body  in  two  principal  ways.  The  first  is  without 


THE    TISSUES   OF  MOTION 


79 


the  use  of  a  rigid  and  jointed  skeleton,  and  the  second  is  with  such  a 
support.  To  understand  how  the  first  method  can  be  used  to  produce 
rapid  and  accurate  motion,  study  the  use  of  muscle  in  the  arm  of  a  squid 
or  octopus,  or  the  proboscis  of  an  elephant.  The  action  of  muscle  with 
a  jointed  skeleton,  is  doubtless  already  understood  by  the  reader  from 
seeing  figures  of  this  operation  in  man  and  in  a  bivalve  mollusk.  As 
the  structure  of  muscle  systems  that  operate  without  a  skeleton  is  often 


I      <~>       I      dx>     i 

c:5      I 

C_)                     C.^2            [       (^~ 

:>     I 

]     c  ^     [      c  2     I      d 

5 

J^  ^       '              <—i 

c^> 

)          | 

i    <t>T   ci     1 

C_J3         | 

\oo>  o 


oo  oo    \ 


\O<O    C3    <O 


OO     OO 


c 

FIG.  79.  —  Diagram  of  several  ways  in  which  muscle  cells  are  arranged  to  form  muscle  tissue. 

a  subject  of  histological  study,  we  shall  briefly  examine  an  example  of 
it  in  a  squid's  arm.  Figure  80  shows  a  line-drawing  of  a  transection  of 
this  organ,  and  the  lettered  bundles  of  muscle  are  used  at  this  point  as 
follows. 

All  muscles  cut  in  cross  section  can  be  used  as  retractors  to  shorten 
the  arm.  The  most  powerful  retractors  are  seen  to  be  in  the  outer  part 
of  the  mass.  They  are  also  used  for  the  partial  contractions  that  bend 
the  squid's  arm.  The  method  of  extension  is  more  difficult  to  see. 
There  being  no  rigid  frame,  the  only  method  is  to  use  some  lateral 
compressions  which  will  force  the  mass  to  extend  in  the  direction  of  the 
arm's  long  axis  and  at  right  angles  to  the  plane  of  the  figure. 

An  examination  of  these  lateral  muscle  bundles  will  show  that  they 
are  confined  to  a  circular,  central  core,  sharply  defined  from  the  outer 
layer  of  longitudinal  fibers  by  a  sheath  of  delicate  connective  tissue. 
This  core  contains  some  small  bundles  of  the  longitudinal  fibers,  but  the 
greater  part  of  its  mass,  outside  of  its  non-contractile  nervous  ganglion 
in  the  center,  is  occupied  by  fiber  bundles  that  lie  in  all  possible  direc- 
tions in  the  plane  transverse  to  the  long  axis  of  the  arm.  When  these 
muscles  contract,  the  central  core  is  compressed  laterally  and  conse- 
quently extended  longitudinally,  while  the  outer  tissues  are,  of  course, 
extended  with  it. 


8o 


HISTOLOGY 


\ 


The  extensory  core  is  placed  at  the  most  efficient  point  in  the  section, 
at  the  center.  Not  that  the  outer  edge  would  not  serve  as  an  extensor 
just  as  well,  but  because  this  outer  edge  must  be  reserved  for  the  bend- 
ing muscles,  which  serve  to  bend,  wave,  and  curl  the  pliable  arms  in  all 
directions.  They  can  do  this,  too,  independently  of  the  state  of  exten- 
sion or  retraction  of  the  arm,  owing  to  their  independence  of  the  core. 
Watch  a  live  octopus  or  squid  and  see  that  its  arm  can  twist  and 

curl,  extend,  or  con- 
tract in  practically 
any  direction.  All 
these  movements  are 
due  to  contractions 
of  muscle  cells,  for 
no  muscle  cell  can 
extend  itself.  This 
must  be  done  for 
it  by  some  other 
counteracting 
muscle  cell  properly 
placed  to  oppose  it. 
Study  the  action 
of  the  muscle  layers 
in  the  intestine  of 
vertebrates  also. 
For  the  origin  of  the 
muscle  tissues,  see 
later  in  this  chapter. 
Several  features 
have  been  used  to 
classify  muscle  cells, 
but  no  two  of  them 
agree  except  within 
very  narrow  limits. 

Their  control  by  the  will  divides  them  into  voluntary  and  involuntary 
muscle  cells,  the  markings  of  the  fibrillse  serve  to  classify  them  as 
striated  and  non-striated  muscle  cells,  the  number  of  their  nuclei 
makes  them  mono-  or  multinuclear,  and  less  important  features  group 
them  into  incomplete  classes,  as  epithelial,  branched,  and  circular 
muscle  cells.  Our  study  may  well  begin  with  the  cylindrical,  multinu- 
cleated,  striated  fiber.  Such  a  muscle  cell  is  universally  found  in 
forms  of  all  grades  where  efficiency  and  economy  is  needed.  It  is  of 
high  specialization  and  is  under  control  of  the  will.  The  example 
taken  is  from  the  common  brook  sucker,  Catostomus  communis. 


FIG.  80. — Transaction  of  squid's  arm;  r,,  retractors;  ex.,  ex- 
tensors; />.,  flexing  retractors;  m.,  membrane  separating  the 
extensor  core  (with  its  retractors)  from  flexors  or  bending  mus- 
cles; n.,  central  nerve  cord.  X  80. 


STRIATED  MUSCLE 


8l 


Muscle  of  Adult  Sucker.  —  A  superficial  examination  shows  that  the 
entire  muscular  body-mass  of  this  fish  is  composed,  on  each  side  of  the 
spine,  of  a  series  of  regularly  bent  plates  or  myotomes,  fitting  closely  to 
one  another  and  joined  sur- 
face to  surface  by  layers  of 
connective  tissue,  or  septa. 
The  myotomes  lie,  as  far  as 
their  shape  allows,  at  right 
angles  to  the  body  axis. 

Under  the  low  power  it 
will  be  seen  that  each  plate 
or  myotome  is  a  mass  of 
fibers  which  stretch  from 
surface  to  surface,  and 
which  lie  exactly  parallel 
with  the  body  axis  and 
consequently  with  each 
other.  They  thus  are  fre- 
quently attached  to  the  sep- 
tum at  a  small  angle. 

Under  the  high  power 
(Fig.  81)  each  fiber  appears 
at  first  sight  to  be  composed 
of  a  series  of  thread-like 
and  regularly  marked  struc- 
tures, the  fibrillae,  which 
run  parallel  to  each  other 
the  entire  length  of  the 
fiber.  Still  closer  attention 
will  show  that  this  mass  of 
fibrillae  does  not  alone  con- 
stitute the  fiber,  but  that  it 
occupies  the  larger  part  of 
the  real  fiber,  which  is  a 
mass  of  sarcoplasm  con- 
taining nuclei  and  bounded 
on  its  surface  by  a  thin, 
tough  membrane,  the  sarcolemma.  The  sarcolemma  adheres  closely  to 
the  entire  surface  of  the  fiber. 

The  sarcoplasm  fills  the  entire  long  cylindrical  muscle  cell  and,  be- 
sides the  prominent  bundles  of  fibrillae  which  lie  in  it,  there  are  also  many 
nuclei  of  a  specific  character  which  distinguish  them  as  muscle  nuclei 
when  compared  with  any  other  nucleus  in  their  locality.  They  are 


FIG.  bi.  —  Longitudinal  section  of  a  bit  of  muscle  from 
the  sucker,  Catostomus.  As  the  scale  will  not  permit 
of  fine  detail,  a  semidiagrammatic  sketch  in  the  lower 
corner  serves  to  show  the  relations  of  dark  and  light 
elements  in  the  three  contraction  stages  of  the  fibril. 
The  chief  figure  shows  the  Q-stripe  separated  as  in  B. 
cap.,  capillaries  containing  blood  cells  and  blood  plate- 
lets; jar.,  sarcoplasm;  mus.n.,  muscle  nuclei;  n.,  con- 
nective-tissue nuclei.  X  1000. 


82  HISTOLOGY 

slightly  elongate,  larger  than  the  average  nucleus  in  other  tissues  of  the 
sucker,  and  have  a  single  or  double  nucleolus  of  large  size  that  stains 
jet  black  in  this  specimen.  The  nucleoli  lie  in  the  longer  axis  of  the 
nucleus,  and  the  caryoplasm  is  clear,  owing  to  the  small  amount  of  the 
chromatin  which  gathers  into  a  few  dense  masses  near  the  nuclear 
membrane. 

The  nuclei  are  fairly  numerous  and  generally  lie  in  the  cytoplasm 
between  the  fibril  bundles  and  the  sarcolemma.  They  here  form  rows 
that  stretch  for  some  distance  in  the  fiber  on  its  long  axis.  In  addition 
to  these  muscle  nuclei  lying  outside  the  fibril  bundles  there  are  several 
which  lie  inside  or  among  the  bundles.  These  are  always  placed  close 
to  the  extreme  end  of  the  fiber,  near  the  septum  of  the  myotome,  and  the 
fibrillae  part  to  pass  on  each  side  of  them,  leaving  a  long  cone-shaped 
space  at  each  end  which  is  occupied  by  sarcoplasm.  The  sarcoplasm 
is  remarkable  for  its  granular  rather  than  its  reticular  appearance. 
This  is  due  to  the  large  number  of  granules,  the  myochondria,  which 
lie  in  irregular  masses  in  an  otherwise  typical  cytoplasm. 

A  delicate  lax  connective  tissue  is  found  throughout  the  mass  of 
muscle  fibers.  Its  individual  cells,  whose  outlines  are  irregular  and 
indeterminate,  possess  nuclei  which  are  characteristic,  being  thin  and 
flat  and  round,  or  elongated  into  an  oval  form.  They  are  smaller  than 
the  muscle  nuclei  and  possess  a  denser  caryoplasm,  which  contains  more 
chromatin.  The  nucleolus  is  multiple  and  its  several  very  small  parts 
are  distributed  around  the  periphery  of  the  nucleus  rather  than  in 
the  center. 

Such  portions  of  this  network  of  connective  tissue  as  come  in  contact 
with  the  cell  body  of  a  muscle  fiber,  form  a  layer  that  covers  every  part 
of  the  fiber's  surface  and  is  so  intimately  connected  with  the  sarcolemma 
that  it  cannot  be  separated  from  it.  The  true  sarcolemma  is  a  cell- wall 
of  homogeneous  structure  and  of  no  great  strength  or  substance.  It 
cannot  be  demonstrated  apart  from  the  connective- tissue  layer  which 
surrounds  and  adheres  to  it.  Such  nuclei  as  are  to  be  in  the  sarcolemma 
belong,  of  course,  to  the  connective- tissue  elements. 

Blood  capillaries,  with  their  delicate  endothelial  walls  and  con- 
tained red  corpuscles,  lie  between  nearly  every  two  fibers.  The  nuclei 
of  their  walls  are  almost  identical  in  shape,  size,  and  other  features  with 
the  connective-tissue  nuclei.  Blood  plates  are  well  demonstrated  in 
these  capillaries. 

The  bundle  of  fibrils  next  claims  our  attention.  That  this  is  com- 
posed of  real  individual  fibrils  is  indicated  by  its  appearance  and  by  the 
fact  that  in  the  specimen  we  examine,  some  individual  fibrils  are  sepa- 
rated from  their  fellows  and  shown  alone  and  in  their  integrity. 

Such  a  fibril  is  composed  of  two  kinds  of  substance  according  to  the 


STRIATED  MUSCLE  83 

staining  and  refracting  power,  and  these  materials  are  distributed  with 
absolute. regularity  and  evenness.  One  of  them,  the  so-called  isotropic 
substance,  which  does  not  readily  stain,  is  probably  responsible  for  the 
tensile  continuity  of.  the  fibril  and  is  also,  probably,  a  less  specialized 
form  of  cytoplasm  than  the  second,  or  anisotropic  substance,  which  is 
doubly  refractive  and  stains  in  our  subject  a  jet  black. 

The  anisotropic  substance  is  deposited,  in  resting  or  relaxed  sucker 
muscle,  in  regular  areas  of  the  fibril,  which  areas  are  of  equal  length 
and  spaced  equally  apart.  Each  of  these  areas  is  designated  for  conven- 
ience by  the  capital  letter  Q.  Such  a  portion  of  anisotropic  substance, 
together  with  one  half  of  the  isotropic  substance  on  each  side  of  it,  is 
called  a  sarcous  element  or  sarcomere.  This  same  arrangement  holds  foi 
all  the  other  fibrils  in  the  sucker's  voluntary  muscle,  and  where  a  number 
of  fibrils  are  grouped  together  in  a  bundle,  the  sarcous  elements  are  all 
in  perfect  alignment  and  directly  opposite  one  another.  This  gives  the 
fiber  a  banded  or  cross-striped  appearance,  from  which  it  gets  its  name, 
striated  muscle  (see  Fig.  81).  These  broad  black  stripes,  as  shown  in  a 
portion  of  resting  sucker  muscle,  are  almost  exactly  two  thirds  the  width 
of  the  intervening  light  stripes. 

In  most  instances  this  band  of  the  sarcous  element  is  divided  at  its 
middle  by  a  lighter  band.  This  is  caused  by  the  beginning  of  the  physi- 
ological act  of  contraction  in  which  broad  black  bands  of  the  sarcous 
elements  separate  into  two  parts.  Figure  81,  A,  shows  this  condition. 

Looking  closely  at  the  light  stripe  which  lies  between  the  broad 
median  band  of  the  sarcous  elements,  we  see  that  it  is  divided  midway 
by  a  black  line  into  two  equal  parts.  This  line  we  shall  call  the  inter- 
mediate septum  (Krause's  membrane}.  Referring  to  one  of  the  com- 
ponent fibrils  again,  it  is  seen  that  this  intermediate  septum  is  repre- 
sented in  the  fibril  by  a  dot  or  spot  which  may  be  called  the  intermediate 
granule.  A  very  close  inspection  under  high  power  will  probably  show 
that  a  transparent  membrane  connects  the  various  intermediate  granules 
into  one  plane,  and  the  whole  structure  forms  the  intermediate  septum, 
which  has  been  called  Krause's  membrane  from  its  discoverer.  This 
is  very  difficult  to  see  in  the  sucker  muscle,  and  may  be  mudi  better 
observed  in  our  next  specimen,  the  lobster's  muscle.  Turning  now  to  a 
section  taken  at  right  angles  to  the  sucker's  muscle  fiber  (Fig.  82),  a 
number  of  points  of  interest  can  be  made  out  to  corroborate  impressions 
formed  while  examining  the  longitudinal  sections. 

The  fibers  are  here  seen  to  be  of  several  sizes,  and  the  continuous 
outline  of  each  sarcolemma  can  easily  be  made  out.  The  connective- 
tissue  nuclei  of  the  sarcolemma  are  sharply  distinguishable  from  the 
muscle  nuclei  which  lie  in  the  sarcoplasm  of  the  fiber  between  the 
sarcolemma  and  fibril  bundles. 


84 


HISTOLOGY 


These  bundles  take  up  the  greater  part  of  the  fiber,  and  the  sarco- 
plasm  lies  between  the  groups  of  fibrillae,  into  which  the  fiber  bundles 
are  divided.  This  grouping  of  the  fibrillae  is  clearly  seen  in  the  cross 
sections  of  sucker  muscle.  The  fibrillae  are  grouped,  near  the  periphery 
of  the  fiber,  into  plates  of  one  fibril  in  thickness.  The  form  of  the 
plates  is  due  to  the  presence  of  a  mass  of  heavier  cytoplasm  that  sur- 
rounds and  binds  them  together.  This  is  the  cement  substance. 

These  peripheral  plates  extend  for  the  same  distance  into  each  fiber, 
whether  it  be  large  or  small,  and  inside  of  that  the  fibrils  are  no  longer 


FIG.  82.  —  Transaction  of  several  related  fibers  of  muscle  in  the  sucker  Catostomus.    }.,  smallest 

fiber. 


a  part  of  the  peripheral  plates,  but  are  either  placed  singly  or  grouped 
into  smaller  independent  plates.  Thus  in  the  smallest  fibers  (as  at  Fig. 
82,  /.),  the  plates  extend  to  the  center,  while  all  larger  fibers  have  the 
center  made  up  of  detached  groups.  It  seems  that  the  fibrils  are  thus 
placed  in  thin  layers  side  by  side,  in  order  that  each  and  every  fibril  may 
be  in  direct  contact  with  some  part  of  the  sarcoplasm  from  which  it 
draws  its  food  and  stimulus,  and  on  which  it  unloads  its  refuse  materials. 

That  the  plates  do  not  extend  in  a  complete  condition  further  inward 
than  they  do  is  probably  due  to  the  requirements  of  the  movement  of 
the  fiber  which  does  not  contract  and  expand  all  its  parts  in  perfect 
unison,  as  will  be  shown  elsewhere. 

The  individual  fibrils  which  show  so  distinctly  in  the  longitudinal 
sections  cannot  be  ordinarily  seen  in  a  transverse  section  of  the  fiber. 
This  may  be  due  to  the  optical  properties  of  the  cement  substance  which 
surrounds  each  fibril  and  which  binds  them  together  in  the  plates.  The 


STRIATED  MUSCLE 


Z 


plates  appear  homogeneous  in  section,  although  at  one  point  it  appears 
that  the  fibrils  are  separately  and  distinctly  shown  in  several  of  the 
plates. 

A  closer  study  of  the  structure  and 
relations  of  the  fibrils  should  now  be 
made  in  some  muscle  whose  elements 
are  more  favorable  for  a  detailed  ob- 
servation. A  suitable  muscle  for  this 
purpose  can  be  found  in  longitudinal 
sections  of  the  muscle  in  a  lobster's  limb 
joint.  Figure  83  is  from  the  basal  joint 
of  the  antenna  in  a  lobster  that  was  on 
the  point  of  emerging  from  its  old  shell. 

This  figure  represents  a  part  of  a 
fiber,  the  upper  end  of  which  was  at- 
tached to  the  shell.  The  two  smaller 
nuclei  are  probably  the  first  edges  of 
nuclei  that  are,  in  reality,  as  large  as  the 
two  larger  ones. 

The  fortunate  condition  is  that  the 
part  of  the  muscle  next  to  the  shell  was 
at  rest  and  in  a  relaxed  condition,  while 
all  the  other  segments  showed  successive 
stages  of  contraction.  The  first  segment 
measured,  in  one  of  its  magnifications, 
17^  mm.  Each  succeeding  one  meas- 
ured a  little  less,  until  the  last  is  only  12 
mm.  long.  This  alone  shows  us  that  we 
have  a  carefully  graded  series  of  contrac- 
tion stages,  and  an  examination  of  the 
appearance  of  the  successive  stages  con- 
firms this  view. 

In  the  first  upper  segment,  of  the 
eight  sarcous  segments  shown  in  any  one 
of  the  sixteen  myo-fibrils  that  compose  this  fibril  bundle,  we  can  notice 
that  the  anisotropic  material  is  deposited  in  a  long  rod-like  area  in  the 
middle  of  the  segment.  This  area  we  will  designate  as  Q,  and  it  is 
usually  so  called  in  works  on  muscle  structure. 

On  each  side  of  Q  is  an  area  of  non-staining  isotropic  substance  not 
more  than  a  quarter  as  long  (or  wide,  if  we  consider  the  whole  band  that 
they  form)  as  Q.  Conforming  with  usage,  we  shall  call  this  ;.  There 
are  two  of  these. 

Bordering  each  j  on  its  side  farthest  from  Q  is  a  row  of  dots  or  round 


FIG.  83.  —  Bit  of  a  muscle  fiber  from  a 
lobster's  antenna.  Partly  contracted. 
Lettering  of  unit  regions  same  as  in 
Fig.  81.  m.,  delicate  membrane  ex- 
tending through  cytoplasm  from  all 
Z  planes.  Apparent  difference  in 
size  of  muscle  nuclei  due  to  tangen- 
tal  sectioning  of  the  smaller  ones. 
conn.t.,  connective-tissue  fibrils  which 
attach  the  muscle  fibrils  to  the  hypo- 
dermal  cell  fibrils. 


86  HISTOLOGY 

bodies.  These  are  of  black  staining  anisotropic  substance  and  are 
called  Z.  They  mark  the  limit  of  the  sarcous  element  or  muscle  unit 
of  the  fibril,  and  where  two  elements  touch,  the  dot  is  double,  although 
the  two  parts  are  molded  together  and  do  not  usually  appear  as  twin 
bodies.  The  upper  segment  in  our  figure  shows  both  a  double  Z  body 
(where  it  touches  the  segment  next  lower)  and  a  single  or  half  Z  body  at 
its  upper  end,  where  it  is  attached  to  the  connective-tissue  cells. 

The  Z  bodies,  then,  form  the  intermediate  line  and  are  connected 
with  one  another  by  a  transparent  or  non-staining  membrane  that  ex- 
tends through  the  cell  from  side  to  side,  even  in  such  parts  of  it  as  contain 
no  myo-fibrils  (see  Fig.  83,  w.).  This  can  be  well  seen  in  the  portion  of 
the  fiber  figured,  especially  where  the  larger  and  smaller  bundles  of 
fibrils  have  separated  to  make  room  for  the  two  large  nuclei  and  their 
surrounding  sarcoplasm. 

The  second  muscle  segment  shows  no  great  difference  from  the  first, 
although  it  is  a  trifle  shorter.  In  the  third,  however,  a  change  has 
occurred  and  Q  has  become  thinner  in  the  middle.  The  whole  segment 
is  about  one  twentieth  shorter  than  the  second.  In  the  fourth  segment 
the  Q  area  is  not  only  thinned  out  in  its  middle,  but  its  substance 
does  not  stain  as  black  in  this  thinned  middle  part. 

The  remaining  five  segments  are  alike  in  the  fact  that  the  Q  areas 
are  elongated  until  their  black  ends  have  met  the  corresponding  ends  of 
the  Q  areas  of  the  neighboring  segments.  In  doing  this  they  hide  the  Z 
dots  and  make  a  much  darker  and  wider  band  across  the  muscle  fiber 
in  its  place.  Their  middle  parts  have  become  clear  and  non-staining 
except  that  an  edge  appears  to  be  left  on  each  side.  This  is  probably 
a  refraction  line. 

In  the  middle  of  this  Q  area,  now  light  and  non-staining,  appears  a 
small  black  dot  that  resembles  the  Z  body  of  the  relaxed  stage  except 
that  it  is  smaller  and  is  not  connected  with  its  neighbors  by  a  mem- 
brane. This  dot  with  its  neighbors  forms  the  M  stripe.  In  Figure  84 
the  M  stripe  is  the  same  width  as  the  two  halves  of  the  Q  stripe. 

The  usual  appearance  of  the  fiber  is  now  entirely  changed,  and  upon 
a  careless  examination  appears  to  have  been  reversed,  as  though  the  Z 
bands  and  Q  bands  had  changed  places.  It  takes  a  careful  eye,  even  in 
such  a  favorable  specimen  as  the  one  from  which  this  figure  was  drawn, 
to  run  from  band  to  band  and  note  the  real  change.  This  difficulty  is  a 
weakness  of  the  eye  muscles  and  can  be  demonstrated  by  an  attempt 
to  count  the  pickets  in  a  distant  fence,  which  can  be  seen  clearly 
if  the  eye  remains  still,  but  are  lost  count  of  if  the  eye  moves  to  follow 
them. 

Many  other  forms  of  striated  muscle  have  a  more  complicated  pattern 
of  sarcous  element  than  the  ones  we  have  been  studying,  as  in  some  in- 


STRIATED  MUSCLE 


~Q 


ncls 


sects,  for  instance,  where  a  band  of  anisotropic  areas  in  the  fibrils  extends 
through  the  middle  of  the  j  area.  When  found,  this  band  is  called  the 
N  band  (Fig.  84).  Its  presence  causes  some  unimportant  complications 
in  the  contraction  processes.  The  isotropic  bands  are  designated  by 
small  letters. 

In  most  muscles  the  bands  are  so  fine  and  close  set  that  it  is  difficult 
to  distinguish  them  clearly.  Then,  again,  one  does  not  know  which  stage 
of  contraction  is  presented.  The  usual  con- 
dition shows  one  of  the  intermediate  stages, 
as  in  the  sucker  muscle,  where  the  eye  of 
the  ordinary  observer  would  take  each  of 
the  two  ends  of  the  divided  Q  stripe  for  the 
Q  stripe  of  a  resting  muscle,  until  study 
showed  the  true  relations. 

Some  sure  way  of  fixing  a  bit  of  muscle 
so  that  it  will  be  certainly  either  relaxed  or 
contracted  would  be  a  useful  method.  A 
muscle  fixed  under  pressure  or  relaxed  does 
not  necessarily  show  its  fibrils  in  either 
condition.  The  hardest  thing  to  get  is  a 
fiber  in  which  there  is  a  slow  contraction 
wave  showing  as  in  Figure  83.  Many  sud- 
den and  contorted  changes  that  are  almost 
useless  for  study  are  usually  found  in  crus- 
tacean muscle  fixed  in  the  ordinary  reagents. 

Technic. — The  element  of  "luck"  ap- 
pears to  be  a  large  one  in  the  preparation 
of  striated  muscle  tissue.  The  best  histo- 

logical  methods,  when  most  carefully  carried  out,  are  pretty  sure  to  give 
bad  results  from  the  point  of  view  of  him  who  wishes  to  study  the  stages 
and  processes  of  contraction.  An  even  more  careful  study,  than  has 
been  made  of  the  conditions  under  which  muscle  may  be  killed  so  as 
to  show  any  particular  stage,  is  most  desirable.  Muscle  which  has  been 
allowed  to  die  a  natural  death  and  which  has  been  killed  with  'chloro- 
form and  in  many  other  ways  should  be  examined.  Animals  should  be 
killed  with  poisons  of  marked  muscular  reactions  and  the  condition  of 
this  tissue  noted.  The  tissue  can  also  be  studied,  to  a  certain  extent, 
while  it  is  yet  alive. 

LITERATURE 

The  works  of  Rollet,  Engelmann,  Retzius,  and  others  should  be  read  by  those  who 
wish  to  go  further  into  the  subject. 


FIG.  84.  — Bit  of  muscle  fiber  from 
the  body-wall  of  an  adult  larva 
of  Corydalis  cornutis.  Shows  the 
(TV)  stripe  near  the  Krause's 
membrane  (Z).  ncl.,  nucleus. 


88 


HISTOLOGY 


THE    HISTOGENESIS    OF    STRIATED    MUSCLE 

Most  striated  muscle  is  developed  from  some  sort  of  external  or 
internal  epithelium,  the  fibrils  being  formed  in  situ  in  the  periphery  or 
on  one  side  of  a  columnar  or  prismatic  cell,  which  may  have  many  posi- 
tions in  the  body. 

In  the  medusa  it  is  the  external,  covering  epithelium  that  develops  the 
myo-fibrils.  These  fibrils  are  formed  in  the  expanded  bases  of  the  cells. 

The  cells  may  lie  on  the  surface 
or  they  may  be  invaginated  into 
grooves,  and  only  the  inner  cells 
of  the  groove  have  the  muscle 
characteristics,  as  in  Figure  85. 
This  represents  a  transection  of 
three  such  invaginated  grooves  in 
the  epithelium  on  the  upper  sur- 
face of  the  body  of  a  Florida  me- 
dusa, Cassiopea  xamachana.  It 
will  be  noticed  that  only  the  one 
or  two  cells  lying  in  the  bottom 
of  the  groove  (which  is  closed) 
have  developed  the  muscle  fibrils, 
which  may  be  seen  in  section  as 
black  dots.  In  a  younger  speci- 
men or  on  the  outer  and  weaker 
surfaces  of  this  same  specimen 
there  would  be  no  invagination, 
and  the  myo-fibrils  would  be  seen  lying  in  the  bases  of  nearly  all  the 
cells. 

The  vertebrate  animals  develop  their  muscle  in  cells  that  were  origi- 
nally a  part  of  the  epithelium  that  lined  the  primitive  coelom.  These 
cells  lose  their  connection  with  the  coelom  when  the  epithelium,  of  which 
they  are  a  part,  is  invaginated  from  the  ccelomic  surface  into  a  number 
of  buds,  which  are  cut  off  and  form  a  row  of  myotomes  or  myomeres  in  the 
sides  of  the  animal.  These  sac-like  masses  flatten  in  a  manner  to  form 
two  plates  and  reduce  the  invagination  cavity  to  a  plane  line.  The  inner 
plate  forms  connective-tissue  elements,  while  the  outer  develops  the 
striated,  voluntary  muscle  of  the  body. 

The  development  of  striated  muscle  in  the  embryo  fish  will  furnish  us 
with  a  good  example.  We  shall  use  an  embryo  sucker,  which  is  easily 
procured  and  prepared. 

A  section  of  a  very  young  embryo  of    about    2  mm.   shows    the 


FIG.  85.  —  Muscle  cells  derived  from  ectoderm 
in  Cassiopea  xamachana.  mes.c.,  mesodermal 
cell;  ect.c.,  outer  ectodermal  cells,  which  prob- 
ably play  but  small  part  in  muscle  formation ; 
mus.c.,  ectodermal  cells  which  are  invaginated 
into  grooves  and  form  muscle  fibrils  in  their 
proximal  cytoplasm.  The  grooves  and  mus- 
cle fibrils  (mus.f.)  are  both  seen  in  transverse 
section. 


HISTOGENESIS   OF  STRIATED  MUSCLE 


89 


outer  or  myogenetic  plate  of  the  myomeres,  each  of  which  consists  of  a 
solid  mass  of  embryonic  cells  with  polygonal  sides,  fitting  closely  to  one 
another  and  not  differing  greatly  in  appearance  from  the  generality  of 
other  embryonic  cells  around  them.  They  are  rapidly  multiplying  at  this 
time,  as  is  demonstrated  by  the  numerous  mitotic  figures  that  are  to  be 
be  seen  (Fig.  86). 

The  first  evidences  of  muscular  differentiation  are  most  marked. 
The  cells  on  the  outer  edge  of  the  myotome  begin  to  lengthen,  pushing 


FIG.  86.  —  A  very  young  myotome  of  an  embryo  of  the  sucker,  Catostomus.  A,  region  of 
mitotic  multiplication  of  sarcoblasts;  B,  region  of  amitotic  multiplication  of  nuclei  in 
young  muscle  cells. 

out  their  cell  bodies  in  a  line  parallel  with  the  main  axis  of  the  fish's 
body.  They  continue  this  lengthening  until  they  reach  from  one  end  of 
the  myotome  well  into  the  mass  of  cells  toward  the  other  end.  Two 
other  features  accompany  this  lengthening  process;  the  cell  becomes 
much  larger  in  bulk  and  its  nucleus  changes  much  in  character,  becom- 
ing larger  and  oval,  while  its  nucleoli  enlarge.  The  staining  reaction  of 
the  nucleus  is  very  different  at  this  time :  the  chromatin  is  spread  out  in 
more  definite  masses  and  the  nucleolus  loses  its  affinity  for  iron  hasma- 
toxylin.  It  will  still  stain  with  it,  but  in  the  decolorizer  it  loses  the 
intense  black  color  long  before  the  nucleoli  of  the  undifferentiated  cells 
do  (see  Fig.  86).  It  should  be  noticed  here  that  the  tissue  represented 
in  Figure  87  is  more  deeply  stained  (i.e.  less  decolorized)  than  that 
shown  in  Figure  86. 


HISTOLOGY 


Immediately  that  this  change  has  set  in,  it  will  be  seen  that  the 
nuclei  begin  to  rapidly  divide.  But  not  mitotically,  as  before.  They 
perform  a  perfect  amitotic  division  that  greatly  increases  the  number 
of  nuclei  without  dividing  the  cell  body.  This  latter  grows  longer  until 
it  reaches  from  the  anterior  to  the  posterior  boundary  of  the  myotome, 
while  its  numerous  nuclei  are  stretched  in  a  single  row  from  one  end  to 

the  other  (Fig.  87).  It  is 
worth  noticing  that  the 
oldest  cells  are  always  on 
the  outer  edge  or  layer  of 
the  myotome  and  hold  that 
position  through  life.  We 
find  the  outer  muscle  cells 
or  sarcoblasts  beginning  to 
perform  the  next  step  in 
development  before  the  in- 
nermost ones  have  changed 
from  mononucleated,  em- 
bryonic cells  into  the 
elongating  and  multinu- 
cleated  sarcoblasts. 

This  next  step  consists 
of  the  formation  of  the 
striated  muscle  fibrils  in  the 
cytoplasm  of  the  sarcoblast 
(Fig.  87).  The  fibrils  ap- 
pear gradually  and  are  faint 

FIG.  87.  —A  later  stage  than  Fig.  86  in  tne  development  fi             o1owlv      crrowino- 
of  muscle  tissue  in  the  sucker,  Catoslomus.     Fibrils  be-  dl         rbl»      '  >lowly       growing 
ginning  to  form  in  the  outer  cells.    No  connective  tissue  more    distinct    as    they    de- 
has,  as  yet,  moved  in  between  the  muscle  cells.     Abun-  i           rpi       J      i       _•      < 
dant  amitotic  division  of  the  nuclei.  Vel°P'    The  dark  amsotropic 

portions  are  to  be  seen  first, 

and,  from  the  length  and  arrangement  of  these  segments,  as  compared 
with  those  of  fully  developed  muscle,  it  seems  that  they  are  laid  down 
and  appear  as  fully  extended  fibrils  from  the  very  first.  They  must,  of 
course,  contract  with  the  rest  of  the  muscle,  although,  being  nearest 
the  center  of  the  body  in  their  weakest  stages,  they  probably  do  not 
have  to  exert  the  contracting  force  to  the  degree  that  the  fibrils  out- 
side of  them  do.  It  should  be  noticed  here  that  the  sarcoblast  can 
contract  before  the  formation  of  fibrils  and  that  they  are  earliest 
mature,  and  first  provided  with  fibrils  that  enable  them  to  contract 
more  strongly,  on  the  outer  edge  of  the  body,  where  the  strength  can 
be  used  to  greatest  advantage  (Figs.  86  and  87).  The  fibrils  appear 
always  in  one  side,  the  inner  side  of  the  cell.  The  nuclei  are,  in 


HISTOGENESIS   OF  STRIATED  MUSCLE 


mu   c 


consequence,  pushed  out  to  the  outer  edge,  which  position  they  occupy 
for  a  long  time  (Fig.  88). 

The  fibrils  are  laid  down  one  after  another,  an  outer  cell  always 
having  one  or  two  more  fibrils  than  a  cell  of  equal  size  just  inside  from 
it.  This  accumulation  of  fibrils  is  continued  until  each  fiber  seems  to  be 
a  mass  of  fibrils  with  a  little  sarcoplasm  clinging  to  it,  rather  than  a  cell 
containing  a  certain  number  of 
fibrils.  For  an  example  of  a 
muscle  cell  that  never  acquires 
many  fibrils,  see  the  heart  of  an 
adult  lobster  (Fig.  92).  While 
the  fibrils  are  appearing  one  by 
one  in  the  outer  cells,  the  con- 
nective-tissue cells  lying  between 
the  epithelium  and  the  myotome 
begin  to  migrate  slowly  in  be- 
tween the  myotomes  where  the 
connective-tissue  septum  is  found 
in  the  adult.  After  they  are  well 
introduced  into  the  septum,  they 
send  cells  down  between  the  sar- 
coblasts,  or  muscle  cells,  as  we 
must  call  them  now,  into  the 
myotome  (Fig.  89).  The  blood 
capillaries  follow  this  connective 
tissue  later;  whether  from  the 
inner  or  outer  edge  was  not  de- 
termined. 

At  this  time  the  muscle  cell  is 
fully  formed,  and  future  changes 
depend  only  upon  its  growth  and 
the  addition  of  more  muscle 

fibrils.  It  is  possible  that  these  cells  or  sarcoblasts  divide.  If  so,  the 
division  is  an  unequal,  longitudinal  splitting  of  the  cell  accompanied  by 
a  proportional  division  of  the  nucleus  and  the  muscle  fibrils.  Figure  82 
shows,  in  transverse  section,  a  possible  division  of  this  sort,  where  the 
smallest  fiber  seems  to  have  recently  split  from  the  next  largest.  In 
fact  the  whole  group  of  fibers,  five  in  number,  seem  to  have  come 
from  one  original  fiber. 

Technic.  —  It  is  comparatively  easy  to  secure  good  preparations  of  this 
tissue  in  its  earlier  stages.  The  tissues  are  soft  and  yield  to  the  best  fixa- 
tives in  a  very  satisfactory  manner.  The  staining  is  also  easy.  Flem- 
ming's  fluid  and  two  or  three  of  the  other  best  methods  should  be  tried. 


mus. 


FIG.  88.  —  Transverse  section  of  a  bit  of  periph- 
eral body  muscle  of  the  sucker,  Catostomus. 
A  little  further  developed  than  in  Fig.  87. 
mu.c.,  mucous  cells  of  skin;  mus.f.,  muscle 
fibrils  in  groups  in  the  proximal  sarcoplasm  of 
the  muscle  cells. 


HISTOLOGY 


LITERATURE 

The  histogenesis  of  striated  muscle  has  been  described  by  several  writers,  among 
whom  are  Eycleshymer,  American  Journal  of  Anatomy,  1902-1903,  and  M.  Heidenhain, 
Anal.  Am.,  1901,  and  J.  B.  MacCallum.  Johns  Hopkins  Hospital  Bull.,  1898. 


CARDIAC   MUSCLE 


In  considering  this  muscle  as  a  class,  we  are  departing  from  all  for- 
mer classification  because  several 
kinds  of  muscle  included  in  the 
other  classes  are  found  in  this.  The 
group,  then,  is  a 
physiological  one, 
not  founded  on  any 
common  structural 
distinction  that  we 
can,  as  yet,  pick  out. 
It  is  usually,  how- 
ever, different  from 
the  other  muscle  tis- 
sues of  the  body. 

The  heart  muscle 
is  distinguished 
physiologically  by 
the  fact  that  it  must 
keep  constantly  in 
action  at  a  con- 
siderable rate  of 
speed  and  tension. 
The  consequent  and 
peculiar  nervous 
and  gross  arrange- 
ments are  a  mor- 
phological and  a 
physiological  matter 
rather  than  a  histo- 
logical  one.  Also, 
as  the  heart  is  else- 
where considered  as 
a  part  of  the  cir- 
culatory channels,  we  shall  pay  attention  here  only  to  the  cytology  of  its 
muscle. 


FIG.  89.  —  Portion  of  peripheral  muscle  tissue 
from  the  embryo  of  a  sucker,  Catostomus. 
Considerably  more  advanced  than  in  Fig. 
88.  ep.,  single-layered  epidermis;  conn.t., 
connective-tissue  cells  migrating  from  the 
subcutaneous  layer  in  between  the  myo- 
tomes  and  thence  in  between  the  muscle 
cells;  bl.ca.,  blood  capillary  containing  red- 
blood  cell  and  blood  platelets ;  bl.c.,  blood 
cell.  From  embryo  of  10.5  mm. 


oo.  —  Two 

:le  cells  from 


FIG. 
muscl 
the  heart  of  Unio. 
Blood  cell 
(wandering  cell) 
attached  to  one. 
XS8o. 


CARDIAC  MUSCLE 


93 


A  smooth  muscle  fiber  with  its  nucleus  and  principal  cytoplasm  body 
lying  outside  of  the  myo-fibril  group  is  described  in  the  wall  of  the  heart 
vessel  of  Cerebratulus.  Other  peculiar  contractile  cells  are  found  in  the 
pulsating  vessels  of  the  worms.  Even  in  these  early  stages  of  phylo- 
genetic  development  we  find  a  cardiac  muscle  cell  that  is  somewhat 
different  from  the  other  muscle  cells  of  the  body. 

In  the  majority  of  the  mollusks  the  heart  muscle  is  composed  of 
smooth  cells.  In  Unio  they  are  simple  spindle-shaped  forms  that  are 


FIG.  91.  —  A,  heart  muscle  tissue  from  the  Gasteropod  mollusk,  Sycotypus.  B,  transections  of 
other  fibers,  one  through  the  nucleus  and  one  a  short  distance  from  it ;  x,  body  of  unknown 
function,  x  700. 

arranged  in  the  peculiar  mesh  work  which  is  met  with  in  most  heart 
muscles  (Fig.  90).  Besides  their  peculiar  arrangement,  they  differ  from 
the  ordinary  Unio  muscle  cells  in  having  less  fibrillar  contractile  material 
and  a  far  larger,  central  mass  of  sarcoplasm.  In  some  mollusks,  espe- 
cially those  of  highest  specialization,  a  well-differentiated  cardiac  muscle 
is  to  be  seen.  It  reaches  its  highest  development  in  the  cephalopods,  but 
the  relations  are  more  easily  demonstrated  in  a  gasteropod,  so  we  shall 
study  the  heart  tissue  of  Sycotypus  canaliculus  (Fig.  91). 

But  little  explanation  is  necessary.  This  tissue  consists  of  cells  that 
are  spindle-shaped  and  contain  the  nucleus  in  the  center  of  the  fiber. 
Like  the  heart  fibers  of  Unio,  they  also  have  developed  the  myo-fibrils  in 
the  peripheral  layer  of  the  sarcoplasm.  The  prominent  difference  is, 
that  the  myo-fibrils  are  striated  by  alternate  areas  of  isotropic  and,  aniso- 
tropic  substance.  They  also  have  more  of  the  fibrils  developed,  giving 
the  cells  a  heavier  and  more  substantial  appearance. 

Striation  is  evidently  a  feature  that  belongs  to  no  particular  set  of 
muscle  cells  but  may  appear  in  any  of  them.  This  view  could  not  be 
held  by  studying  the  mammalian  body  alone.  The  striation  is  some- 
times hard  to  find  in  Sycotypus.  One  may  examine  many  differently 
prepared  sections  and  see  no  signs  of  it  until,  at  last,  the  right  part  of  the 
right  one  will  show  it  clearly  and  indubitably.  The  striations  are  more 
easily  demonstrated  in  the  squid's  heart. 


HISTOLOGY 


Many  undoubtedly  smooth  fibers  have  their  fibrils  lying  in  a  waved 
position  that  simulates  striation.  This  is  the  more  easily  seen  because  the 
angles  of  such  waves  hold  quantities  of  the  stain  in  a  crude  physical  way, 
especially  iron  haematoxylin.  In  this  connection  we  shall  mention  the 
unique  case  of  the  muscle  cell,  described  by  Schaper  from  the  salaman- 
der's mesentery,  which  showed  alternate  light  and  dark  bands.  (As 
Schaper's  description  deals  with  the  in- 
dividual myo-fibrils,  however,  we  must 
accept  it  as  a  remarkable  exception.) 

In  the  Crustacea  and  Insecta  the  dif- 
ferentiation of  cardiac  muscle  is  more 
complete  than  in  any  lower  forms.  In 
the  insect  it  is  precisely  the  same  as  the 
other  body  muscles,  except  the  unimpor- 
tant difference  of  having  slightly  smaller 
fibers  and  slightly  narrower  striations. 
This  is  pictured  in  Figure  141. 

In  the  lobster  we   find  a  case  that  is 
much   like    that    of    the    vertebrates,    an 
apparent    syncytium,    in    which   striated 
myo-fibrils   are   developed   (Fig.  92).     Its 
bodies  cf  unknown  meaning.  One    embryology  would  probably  show  a  multi- 

of  these  lies  in  close  contact  with  a          „    ,  ...  ,  .      .  , 

nucleus ;  /  marks  two  of  the  fibril    cellular  origin,  but  as  this  is  not  known, 
we  shall  study  the  adult  form. 

Any  section  shows  a  great  wealth  of 
fibers  running  in  all  possible  directions. 

This  apparent  lack  of  aim  in  placing  cardiac  fibers  in  any  particular 
direction  has  been  mentioned  previously  and  has  its  mechanical  advan- 
tages. The  fibers  are  not  individual  so  far  as  boundaries  can  be 
detected,  but  form  strands  of  one  large  syncytium. 

The  sarcoplasm  is  very  abundant  in  proportion  to  the  other  contents 
present  and  contains  three  prominent  objects,  nuclei,  myo-fibrils,  and 
characteristic  bodies  of  doubtful  function. 

The  nuclei  are  fine  and  large,  as  are  most  of  those  in  the  lobster's 
tissues.  Like  other  lobster  nuclei,  they  have  a  delicate  but  rigid  nuclear 
membrane,  finely  distributed  chromatin,  and  a  small  but  very  clearly 
defined  nucleolus.  In  a  transection  of  almost  any  single  connecting 
strand  there  are  to  be  seen  from  one  to  three  nuclei,  more  rarely  none 
or  more  than  three.  Figure  92  shows  a  typical  transection. 

The  myo-fibrils  are  especially  worth  study  in  such  a  section  as  the 
above  because  they  are  comparatively  few  in  number  for  such  a  well- 
differentiated  muscle  cell  and  serve  to  do  away  with  the  idea,  so  preva- 
lent among  beginners  who  study  only  human  histology,  that  a  muscle  fiber 


t.  n. 

FIG.  92. — Transection  of  a  single 
muscle  fiber  in  heart  of  the  lobster. 
Shows  two  nuclei,  about  twelve 
fibril  bundles  and  four  of  the  x 


bundles;  conn.t.n.,  connective-tis- 
sue nucleus  or  sarcolemma  nu- 
cleus. 


CARDIAC  MUSCLE  95 

is  a  series  of  bundles  of  myo- fibrils  in  which  some  sarcoplasm  and  nuclei 
have  accidentally  become  entangled  rather  than  a  cell  with  all  its  normal 
organs,  that  contains  in  addition  some  myo-fibrils. 

Our  illustration  (Fig.  92)  shows  a  large  developed  cell  that  con- 
tains about  fifteen  small  bundles  of  the  fibrils.  These  bundles  are 
subdivided  into  smaller  bundles  and  the  subdivisions  contain  on  a  very 
rough  estimate  about  thirty  fibrils  on  an  average.  The  total  cross-section 
area  of  the  bundles  would  not  be  a  third  of  that  of  the  whole  cell. 

The  sarcoplasm,  like  that  of  most  muscle  cells,  is  granular.  But 
scattered  at  frequent  intervals  through  the 
cell  are  peculiar  chromatic  bodies,  spike- 
shaped,  with  the  blunt  end  about  as  large 
as  a  cardiac  nucleolus.  Each  one,  where  it 
lies  in  the  sarcoplasm,  is  surrounded  by  a 
clear  zone  that  is  visible  in  the  figure.  Fig- 
ure 92  shows  four  of  these  bodies,  two  seen 
from  above  and  two  seen  from  the  side.  One 
of  these  latter  lies  very  close  to  a  nucleus. 
The  similar  bodies  in  the  Sycotypus  heart 
muscle  were  round  instead  of  spike-shaped. 

The    fiber   is    everywhere    covered    by   a  F'G-  93-  -Several  muscle  cells 

J  J  from  the  human  heart  showing 

clearly  marked  sarcolemma,  a  connective- tis-  the  "  intercalated  disks "  where 
sue  sheath  that  is  made  of  cells,  and  con-  the  bodies  touch  each  other- 

.  Nuclei  not  shown.     (After  M. 

sequently  has   its  own  nuclei.  HEIDENHAIN.    From  STOHR'S 

A  lateral  view  of  one  of  these  fiber  por-  "Text-book of  Histology"  by 
tions  will  show  several  important  features. 

The  fibril  bundles  are  practically  continuous,  running  from  one  mesh 
to  another  of  the  reticulum,  sometimes  dividing  and  merging  their 
fibrils  with  other  bundles. 

The  fibrils  are  beautifully  striated,  and  although  it  is  sometimes  hard 
to  see  the  striations,  they  can  be  seen  in  many  stages  of  contraction. 
The  pattern  of  striation  is  the  same  as  that  of  the  other  lobster  muscle, 
except  that  the  segments  are  much  shorter,  and  the  striations  conse- 
quently much  finer  and  more  closely  set.  The  peculiar  cells'  found 
among  the  muscle  fibers  are  possibly  excretory  in  function. 

The  vertebrate  heart,  as  exemplified  by  the  heart  of  man,  probably 
shows  the  most  highly  specialized  cardiac  muscle  tissue,  unless  that  of  the 
insects  can  be  so  considered  on  account  of  its  very  perfect  muscle  fibers. 

The  human  cardiac  muscle  is  called  a  syncytium,  although  it  originates 
as  a  mass  of  mesenchymal  cells  with  their  bodies  irregularly  in  contact 
so  as  to  form  a  thick-meshed  reticulum.  In  common  with  the  lobster's 
heart,  its  striated  fibrils  run  continuously  through  the  reticulum,  regard- 
less of  cell  boundaries. 


96  HISTOLOGY 

And  yet  in  the  human  tissue  this  continuity  of  the  fibrils  is  only  func- 
tional, the  fibrils  being  broken  at  the  cell  boundaries  by  the  interposition 
of  small  (probably  non-contractile)  intercalated  disks  (Fig.  93).  We 
must  remember  that  where  two  cells,  provided  with  fibrils  developed  to 
sustain  a  strain,  join  each  other  in  the  line  of  that  strain,  the  fibrils  of 
each  must  join  with  those  of  the  other,  or  they  will  not  be  able  to  perform 
their  function.  They  would  then  soon  atrophy  from  disuse.  Read 
the  discussion  of  the  lobster's  ligament  tissue  for  its  bearing  on  the 
fibrLlar  continuity  of  joining  cells  (see  Fig.  67  and  description). 

Each  fibril,  in  the  distinct  individual  cells  of  the  mammalian  heart 
muscle,  joins  the  fibrils  of  an  adjoining  cell  in  order  to  have  a  strong  and 
functional  point  of  attachment.  It  is  very  doubtful  if  this  muscle  cell 
supports  any  but  its  own  fibrils  trophically,  or  even  furnishes  them  a  nerv- 
ous stimulus,  although  this  latter  case  is  more  probable  than  the  first. 

Each  cell  is  possessed  of  a  single  large  nucleus,  as  a  rule,  although 
there  may  be  two.  In  many  cases  the  appearance  leads  one  to  think  that 
an  amitotic  division  is  taking  place,  which  is  not  probable.  The  cells 
are  branched  at  an  acute  angle,  and  by  joining  their  short  processes  with 
the  other  muscle  cells  they  form  the  muscular  reticulum.  The  cell 
boundaries  are  sharply  marked  by  the  rows  of  "  intercalated  disks," 
which  so  resemble  one  of  the  striations  that  unless  stained  specially  they 
are  not  easily  seen.  A  connective-tissue  sarcolemma  invests  all  parts  of 
the  cardiac  fiber  reticulum.  Its  narrow,  dark  nuclei  form  a  sharp  con- 
trast to  the  full-bodied  oval  nuclei. 

The  striation  of  this  muscle  bears  the  same  relation  to  the  body  muscle 
in  man  that  the  two  bear  in  the  lobster;  it  is  the  same  in  structure  but 
much  finer.  Because  of  this  it  is  sometimes  a  little  hard  to  demonstrate. 

About  the  only  two  generalizations  that  we  can  extract  concerning 
the  cardiac  muscles  are :  first,  the  cells  form  an  irregular  reticulum  which 
can  be  explained  on  mechanical  grounds ;  and  secondly,  the  striation  or 
segmentation  of  the  fibrils  is  finer  than  that  of  the  other  body  muscles. 
The  latter  feature  probably  has  some  unknown  physiological  significance. 

Technic.  — The  technic  of  cardiac  muscle  hardly  differs  from  that  of 
ordinary  striated  muscle.  The  tissue  is  a  little  more  apt  to  become  brittle 
and  a  little  more  difficult  to  stain.  The  intercalated  disks  are  brought 
out  by  the  use  of  nitrate  of  silver. 


LITERATURE 

MACCALLUM,  J.  B.     "  On  the  Histology  and  Histogenesis  of  the  Heart-muscle  Cell," 

Anal.  Anz.,  1897. 
HEIDENHAIN,  M.     "  Uber  die  Structur  des  menschlischen  Herzmuskels,"  Anat.  Anz., 

1901. 


SMOOTH  MUSCLE  97 


SMOOTH    MUSCLE   TISSUES 

Smooth  muscle  originates  in  the  embryo  as  a  specialization  of  mesen- 
chymal  cells.  It  appears  as  a  set  of  unicellular  fibers,  all  lying  parallel 
for  mutual  support  in  their  efforts.  Sometimes  they  are  crossed  or  inter- 
laced, but  in  this  case  only  certain  sets  act  together  to  effect  certain  mo- 
tion. All  the  fibers  in  one  direction  may  contract  to  cause  a  shortening 
of  some  part  of  the  body ;  again,  all  the  fibers  lying  in  several  directions 
in  one  plane  or  in  two  given  planes  may  contract  to  force  an  extension 
of  a  part  of  the  body  in  another  direction  or  plane.  In  form,  these  fibers 
are  almost  always  elongated  spindles  tapering  into  thin  ends.  Very 
rarely  these  ends  may  be  branched  into  two  or  three  main  branches,  as  is 
sometimes  found  in  the  young  mammalian  aorta,  or  they  may  give  off 
many  fine  side  branches,  as  in  the  smooth  muscle  cells  of  the  heart  of 
some  mollusks.  These  side  branches  are  not  contractile,  however. 

The  smooth  muscle  fiber  is  always  formed  in  a  single  cell  and  has  a 
single  nucleus.  We  recall  no  case  where  it  has  the  syncytial  multiplicity 
of  nuclei  found  in  the  large  striated  muscle  cells.  This  nucleus  may 
appear  inside  of  the  fiber  or  outside  of  it.  This  idea  may  be  as  well 
expressed  by  saying  that  the  myo-fibrils  are  distributed  around  the 
nucleus  or  to  one  side  of  it. 

The  smooth  muscle  cell,  like  all  muscle  cells,  owes  its  contractile 
power  to  the  development  of  a  varying  number  of  myo-fibrils  in  its 
cytoplasm.  These  fibrils  may  be  very  few  and  appear  unimportant  in  the 
make-up  of  the  cell,  or  they  may  be  many,  and  at  first  sight  be  all  of  it. 
Perhaps  the  most  ordinary  method  of  their  appearance  is  in  the  entire 
periphery  of  the  cell  where  they  form  a  thin  layer  (see  Fig.  97),  or  a  thick 
layer  that  occupies  most  of  the  room  in  the  cell  body  (see  Fig.  99).  When 
the  fibrils  appear  in  bundles  or  a  single  bundle  in  one  side  of  the  celk 
body,  their  development,  if  large,  leaves  the  cell  body  as  an  apparent 
appendage  on  one  side.  This  is  seen  in  some  low  forms,  especially 
well  in  the  nematode  worms  (see  Fig.  95). 

The  smooth  muscle  fiber  is  generally  used  by  animals  that'  move 
slowly  or  in  parts  of  the  anatomy  of  more  active  animals  where  a  slower 
motion  not  directly  controlled  by  the  will  is  needed.  There  are  a  few 
examples,  however,  of  organisms  which  are  noted  for  their  swiftness  and 
beautiful  muscular  activity,  and  yet  have  nothing  but  smooth  muscle 
to  perform  their  actions  with.  The  squid  is  such  an  example,  and  its 
muscle  is  also  absolutely  under  the  control  of  the  larger  and  central 
nerve  centers,  making  it  "voluntary"  in  a  proper  sense. 

A  contractile  fiber  from  Euspongia  officinalis  will  show  a  very  low 
form  of  smooth  fiber  with  very  few  fibrils  and  much  granular  sarcoplasm. 


HISTOLOGY 


This  structure  has  but  weak  powers  of  contraction  and  might  be  con- 
sidered as  half  connective  tissue  rather  than  muscle  (Fig.  94). 

Cerebratulus  shows  an  interesting  form  of  fiber  in  which  the  fibrils 


FIG.  94.  —  Two  contractile  fibers  from  Euspongia.    (From  SCHNEIDER  after  F.  E.  SCHULTZE.) 

are  few  and  in  another  part  of  the  cell  from  the  nucleus.  See  the  descrip- 
tion of  the  blood  vessels  in  this  form  for  descriptions  and  illustration 
(Chapter  XII). 

The  longitudinal  muscle  fibers  of  A  scar  is  megalocephala  show  a  very 
highly  specialized  cell  in  which  the  nucleated  cell  body  is  situated  on 

the  inner  side  of 
its  plate-shaped 
muscle  bundles. 
The  bundles  con- 
sist of  a  single 
row  of  fibers  em- 
bedded  in  a 
cement  substance 
of  considerable 
thickness.  The 
cell  body  sends 


muz.  fi,  b. 
f\ 


FIG.  95.  —  Transection  of  body- wall  muscle  cell  of  an  A  scar  is  from 
the  cat's  pyloris  ;  nu.,  nucleus  ;  mus.fi.b.,  muscle  fiber-bundles  in 
transverse  section.  X  1000. 


off  some  peculiar 
processes,  one  of 
which  is   said   to 
Figure   95   was  drawn  from  the  common 


have  a  nervous  function. 
Ascaris  in  the  cat. 

The  gizzard  of  the  earthworm  is  provided  with  a  covering  of  very  well- 
developed  and  closely  packed  fibers  of  rather  short  length.  Their  sides 
are  flattened  from  close  contact  with  their  fellows  and  the  edges  are 
sharp.  The  outer  edge  is  flat  and  the  inner  edge  sharp,  the  whole  effect 
being  that  of  a  forged  knife  blade  at  each  end.  One  end  of  such  a  fiber 
is  seen  in  Figure  96. 

The  body  of  the  fiber  is  composed  of  a  very  coarsely  granular  sarco- 
plasm,  containing  a  great  many  thick  myo-fibrils  that  run  in  slightly 
curving  lines  from  end  to  end.  The  fibrils  are  thickest  near  the  surfaces, 
and  only  in  the  interior  of  the  fiber  is  it  possible  to  see  the  sarcoplasm. 
It  occurs  here  in  several  long,  spindle-shaped  masses,  in  the  largest  of 
which  lies  the  nucleus.  This  nucleus  is  oval  and  not  specialized  in  any 
particular  way.  It  is  not  even  much  elongated,  as  many  such  nuclei 
would  be  under  similar  conditions. 


SMOOTH  MUSCLE  99 

A  sarcolemma  has  not  been  demonstrated.  Its  presence  in  this 
muscle  would  demonstrate  the  presence  of  a  real  sarcolemma  on  a 
muscle  cell  on  account  of  the  small  amount  of  connective  tissue  in  the 
organs  of  the  earthworm.  A  notable  feature  of  this  cell  is  the  series 
of  fine  fibrils  that  are  given  off  from  its  body  to  connect  it  with  the  other 
similar  muscle  cells  with  which  it  is  in  contact.  These  connections  must 


fi 


FIG.  96.  —  End  of  a  single  muscle  fiber  from  the  gizzard  of  Allolobophora.  Teased  preparation. 
gr.cyt.,  granular  cytoplasm;  nu.,  nucleus;  conn.fi.,  connecting  fibrils  broken  by  testing; 
fi.,  muscle  fibrils.  X  435. 

be  of  considerable  strength,  as  the  muscle  could  not  hold  together  and 
operate  if  they  were  not. 

These  intercellular  connecting  fibrils  are  a  part  and  product  of  the 
muscle  cell,  and  we  may  say  that  this  cell  forms  and  uses  both  myo-fibrils 
and  connective-tissue  fibrils.  They  are  placed  in  short  rows  and  appear 
to  emerge  from  the  body  of  the  cell  between  the  myo-fibrils.  Seen  with 
a  lower  power,  they  lead  one  to  think  that  the  muscle  cell  is  a  striated 
one.  They  are  broken  off  a  short  distance  from  the  surface  of  a  macer- 
ated fiber  where  the  neighboring  fiber  was  torn  away.  Their  presence, 
under  the  high  power,  gives  the  whole  cell  a  rough  or  prickly  appearance. 

Such  structures  do  not  represent  any  vital  connection  between  the 
cells,  not  being  protoplasmic  in  nature.  They  cannot,  therefore,  be 
called  "protoplasmic  bridges,"  and  should  not  be  called  intercellular 
bridges,  as  that  has  come  to  mean  the  same  thing. 

The  muscle  fibers  of  the  squid  should  be  briefly  compared  with  the 
one  we  have  just  examined.  This  muscle  cell  is  of  extreme  length  and 
is  attached,  where  it  meets  with  a  surface,  by  a  blunt  end;  or  where  it 
is  interlaced  with  other  fibers,  by  a  long,  thin  ending.  Its  myo-fibrils 
are  massed  on  the  surface  of  the  fiber  in  a  thin  layer,  thus  leaving  a 
large  cavity  in  which  the  watery  sarcoplasm  lies. 

The  nucleus  occupies  the  central  part  of  this  cavity,  and  is  enormous 
compared  with  the  nucleus  of  the  earthworm's  gizzard  cell.  It  makes 
a  beautiful  object  for  the  study  of  the  nuclear  organs.  Figure  97  shows 
several  transverse  sections  and  a  longitudinal  section  of  such  fibers  in  the 
squid's  arm. 

The  powerful  closing  muscles  of  the  plecypod  mollusks  show  a  fiber 


IOO 


HISTOLOGY 


that  is  unicellular  and  spindle-shaped,  and  has  all  the  characteristics  of 
smooth  muscle  except  one  (Fig.  98).  When  contracted,  it  shows,  not  a 
transverse  striation,  but  a  peculiar  series  of  oblique  striations,  little  groups 
of  which  pass  each  other  at  an  angle.  The  exact  nature  of  this  striation 


FIG.  97.  — A,  longitudinal  sections  of  several  muscle  fibers  from  the  squid,  Loligo;  B,  trans- 
verse sections  of  the  same  at  different  levels.     X  700. 

has  not  been  determined  upon,  and  so  it  cannot  be  compared  with  that 
of  ordinary  striated  muscle.  So,  also,  the  cell  itself  cannot  be  accurately 
compared  with  other  unicellular  and  spindle-formed  muscle  cells.  Both 
this  last  form  and  that  of  the  squid  are  "voluntary  fibers." 


FIG.  98.  —  Portions  of  two  longitudinal  sections  of  fibers  from  the  closing  muscle  of  Venus. 
«.,  nucleus.     X  870. 

The  involuntary  muscle  cell  of  the  vertebrates  will  form  our  next  and 
last  example  of  this  class  of  muscle  fiber.  This  resembles  the  squid's 
fiber  in  that  its  myo-fibrils  are  laid  down  in  the  peripheral  sarcoplasm. 
It  shows  more  of  them,  however,  so  many,  that  but  little  sarcoplasm  can 


FIG.  99.  —  A,  longitudinal  section  of  several  smooth  muscle  fibers  in  the  bladder  of  a  calf; 
conng.fi.,  connecting  fibrils  between  the  cells;  B,  transverse  section  of  similar  fibers. 

be  found.  What  can  be  seen  lies  at  either  end  of  the  rather  elongate 
nucleus.  This  cell  has  its  own  variations  in  the  different  forms  of  verte- 
brates, and  our  example  is  taken  from  a  section  of  the  bladder  of  a  calf 
(Fig.  99).  In  some  other  forms  the  nucleus  is  much  longer  and  thinner, 
and  can  be  seen  to  twist  and  curl  when  the  muscle  contracts.  The 


SMOOTH  MUSCLE 


101 


largest  of  these  fibers  is  seen  in  the  pregnant  uterus  of  mammals  at  term, 
when  the  fibers  attain  an  enormous  size. 

The  smooth  muscle  cells  in  the  walls  of  the  calf's  bladder  show  an 
intercellular  fibrous  connective  tissue.  This  can  be  differentiated  from 
the  muscle  substance  by  staining,  and  it  appears  as  a  fibrillar  material 
in  our  figure.  It  is  produced,  as  was  the  similar  substance  of  the  earth- 
worm's gizzard  cell,  by  the  muscle  cell. 

A  section  of  any  tubular  portion  of 
the  digestive  tract  of  a  mammal  em- 
bryo of  the  right  age  will  show  the 
origin  of  smooth  muscle.  Lewis  has 
described  the  process  in  the  esophagus 
of  a  pig  of  8  mm.  (Fig.  100).  The 
organ  consists  at  first  of  an  inner 
stratified  epithelium  resting  on  a  mes- 
enchymal  layer  of  primitive  connective 
tissue  whose  cells  have  not  yet  begun 
to  form  the  fibrils.  When  the  fibrils 
do  appear,  they  are  myo-fibrils,  all 
lying  in  the  same  direction  and  filling 
the  cells  up  until  they  are  the  familiar 
smooth  muscle  cells. 

These  cells  do  not  appear  through- 
out the  mesenchyme,  but  in  two  layers 

Of  it,  the  inner  of  which  produces  fibers 

that  run  in  a  circular  direction,  while 
the  outer  one  lays  them  down  at  right 
angles  to  these  and  parallel  to  the  axis  of  the  tube.  Both  these  layers 
are  removed  from  the  epithelium  by  a  third,  which  remains  a  true  con- 
nective tissue,  becoming  a  layer  of  soft,  fibrous  connective  tissue  in  the 
adult  state.  Even  while  forming  the  muscle  fibers,  the  cells  remain 
partly  connected  by  processes  which,  at  this  stage,  are  true  intercellular 
processes.  This  clearly  appears  in  the  transections  of  the  longitudinal 
or  outer  layer  of  muscle  cells,  and  these  bridges  form  the  fine  'inter- 
cellular connective  tissue,  afterwards  retreating  into  the  cell  and  becom- 
ing part  of  its  sarcoplasm. 

Technic.  —  Smooth  muscle  differs  from  the  other  kinds  only  in  the 
way  that  several  special  processes  can  be  applied  to  it.  Its  cells  are 
most  easily  isolated  by  maceration  and  teasing,  even  after  they  have 
been  fixed  in  several  fluids.  Such  preparations  should  be  supplemented 
by  well-stained  and  thin  sections  in  which  the  finer  cytological  details 
can  alone  be  brought  out.  There  are  stains  that  are  specific  for  the 
smooth  muscle  fiber  when  there  is  doubt  as  to  whether  it  is  muscle  or  a 
connective-tissue  structure  (see  LEE). 


a  transverse  section  of 
the  digestive  tract  of  an  embryonic  pig. 
mus.fi. ,  developing  smooth  muscle  fibers. 
(From  "  STOHR'S  Histology,"  by  LEWIS.) 


IO2 


HISTOLOGY 


LITERATURE 

SCHAFFER,  J.   "Zur  Kentniss  der  glatten  Muskelzellen  insbesondere  ihrer  Verbindung," 

Zeits.  f.  Wiss.  Zool.,  Band  LXVI. 
ARNOLD,  J.  "  liber  Structur  und  Architectur  der  Zellen.  3,  Muskelgewebe,"  in  Arch.  f. 

mik.  Anat.,  Band  LII. 


UNUSUAL   FORMS   OF   MUSCLE 

Muscle,  as  has  been  said,  must  be  developed  wherever  needed.     Hav- 
ing accounted  for  some  of  the  usual  places  and  methods  of  formation, 

we  find  a  few  very  unusual 
forms  which  cannot  be  brought 
under  the  other  classifications. 
An  example  of  such  a  pe- 
culiar muscle  cell  is  to  be  seen 
in  the  large  epithelial  cell  that 
lies  next  to  the  water  pore  in  a 
sponge,  Leucosolenia(Fig.ioi). 
The  edge  of  this  cell  is  trans- 
formed into  a  fiber-bundle  that 
surrounds  the  pore,  and,  by 
contracting  or  relaxing,  it  reg- 

FIG.  ioi.  — Part    of    the    body    wall    of  a  simple     ulateS  the  flow  of  Water.       The 
sponge,  LeucosoLenia.    f.vac.,  food  vacuoles  in  en- 
dodermal  cells  ;  mus.c.,  muscle  cells  surrounding 
the  water  pore  (p.);  spc.,  spicule.     X  900. 


outline  of  this  cell  is  not  to  be 
seen,  but  is  undoubtedly  a  defi- 
nite outline  and  perhaps  a 
It  could  be  brought  out,  per- 


regular  one,  like  most  epithelial  cells, 
haps,  by  the  use  of  silver  nitrate 

Another  muscle  is  peculiar  from  the  remarkable  way  in  which  its 
cell-body  is  separated  from  the  myo-fibrils.  This  is  seen  in  the  muscle 
of  the  parasitic  Cercaria  from  Helix.  The  cell-body  of  this  muscle 
cell  lies  entirely  apart  from  the  fibrils  and  sends  to  each  one  a  single 
strand  of  cytoplasm  to  support  it  (Fig.  102).  It  is  probable  that  each 
strand  covers  all  of  the  fibrils  that  it  goes  to,  on  account  of  the  needs  of 
trophic  and  functional  support.  A  fibril  cannot  act  alone  if  any  of  our 
conceptions  of  muscle  activity  be  near  the  truth.  The  fibril  must  have 
some  portion  of  sarcoplasm  in  contact  with  it  to  furnish  it  (according  to 
Englemann's  theory)  with  the  requisite  heat-oxydization  that  causes  it 
to  swell  and  shorten. 

The  stalks  of  some  Protozoa,  as  Vorticella,  are  capable  of  a  very  strong 
and  rapid  contraction  (Fig.  103).  This  seems  again  to  be  a  case  where 
protoplasm  cannot  contract  with  sufficient  efficiency,  but  yet  is  capable 
of  developing  an  arrangement  which  can  so  contract  for  it.  The  fibril 


PECULIAR  FORMS   OF  MUSCLE 


103 


in  the  stalk  is  a  product  of  the  thin  layer  of  cytoplasm  that  surrounds 
and  nourishes  it. 

The  fibril  is  not  like  the  ordinary  muscle-fibril  in  shape  and  mode  of 
action.  It  is  very  large  and  heavy,  and  when  it  contracts  it  does  so  by 
contracting  one  of  its  sides.  As,  according  to  Entz,  this  side  forms  a 


FlG.  102. — Muscle  cell  of  Cercaria  from  Helix,   mus.c.,  muscle  cell  ;  mus.fi.,  muscle  fibrils. 
(After  BETTENDORE.) 

spiral  band  around  the  fibril,  the  result  is  that  the  fibril  is  thrown  into 
a  spiral  shape  resembling  a  wire  bed  spring.  Our  specimens,  and  con- 
sequently the  illustration,  did  not  show  this  spiral  band. 

The  cilia  and  flagella  found  on  many  cells  all  through  the  animal 


FIG.  103. — Proximal  portions  of  two  Vorticella  showing  the  insertion  and  upper  parts  of  the 
contractile  stalks.     X  800. 

series,  are  one  of  the  most  primitive  forms  of  motion  that  affect  the  cell's 
relations  with  the  exterior  (see  other  figures). 

These  organs  are  probably  passive  rods  of  various  lengths  and  thick- 
nesses which  are  moved  by  the  cytoplasm  in  or  near  the  edge  of  the  cell. 
They  are,  apparently,  continuations  of  fibril-like  areas  inside  the  cell. 
This  last  connection  has  been  used  to  liken  the  cell  fibrils  of  the  cilia  to 
the  rays  of  the  centrosome,  which  are  also  organs  of  motion,  and  to  come 
back  to  the  centrosome  as  the  fundamental  cell-organ  of  motion. 


IO4  HISTOLOGY 

While  attractive,  this  hypothesis  is  not  grounded  upon  sufficient 
knowledge  and  should  not  be  too  seriously  entertained. 

Technic.  — The  sponge  material  had  best  be  fixed  in  pure  watery 
solution  of  sublimate  and,  after  the  mercury  compounds  have  been 
taken  out  with  potassium  iodide,  stained  with  weak  alum  carmine 
progressively,  that  is,  stained  slowly  with  a  very  weak  solution  of  the 
stain,  so  that  a  subsequent  decolorization  is  not  necessary. 

LITERATURE 

ENTZ,  G.     "Die  elastischen  und  contractilen    Elemente  der   Vorticellinen,"  Math.  u. 
Naturwiss.  Berichte  a.  Ungarn,  X,  1-48. 


CHAPTER   IX 
ELECTRIC  TISSUES 

IN  a  very  few  organisms,  certain  tissues  are  able  to  produce  electricity. 
They  are  especially  developed  and  constructed  to  do  this,  and  they  pro- 
duce it  specifically,  and  apart  from  the  electricity  generated  in  small 
quantities  as  a  by-product  in  some  other  tissues.  These  few  animals 
are  all  fishes,  some  teleosts,  some  elasmobranchs. 

Electric  tissue  is  composed,  in  the  few  known  cases  where  it  occurs, 
of  a  series  of  plate-like  units,  each  of  which  may  be  designated  by  the 
name  electroplax.  Each  electroplax.  lies  in  a  connective-tissue  compart- 
ment, imbedded  in  a  jelly-like  mass  of  tissue  which  fills  the  compartment. 
The  nerve  and  blood  supply  come  from  some  side  or  corner  of  the  com- 
partment and  are  distributed  through  the  jelly  tissue  to  the  electroplax. 
These  plate-like  electroplaxes  are  arranged  in  rows  or  are  irregularly 
massed.  All  the  electroplaxes  in  a  given  fish  are  oriented  alike. 

The  electroplax  may  be  considered  to  be  a  single  cell  with  many 
nuclei  or,  in  other  cases,  as  a  syncytium  formed  by  the  union  of  a  number 
of  cells.  Some  might  consider  it  an  organ  unit  composed  of  many 
cells  on  account  of  the  fact  that  each  nucleus  is  surrounded  by  its  own 
portion  of  unspecialized  cytoplasm,  but  it  can  probably  be  considered 
better  as  a  syncytium  in  the  same  sense  that  a  voluntary  muscle  fiber  is 
so  considered. 

The  electroplax  is  composed  of  three  principal  layers,  a  nervous  or 
electric  layer  forming  one  surface  on  which  the  nerve  ends,  a  middle 
layer  which  may  be  called  the  striated  layer,  and  a  posterior  layer  whose 
exact  function  is  not  known,  but  may,  perhaps,  be  a  nutritive  layer.  This 
layer  is  really  a  part  of  the  anterior  or  electric  layer,  and  is  continuous 
with  it  around  the  edge  of  the  middle  layer.  In  fact,  the  nerve  some- 
times passes  through  or  around  the  entire  electroplax,  and  turns,  to  branch 
out  and  innervate  the  posterior  layer  instead  of  the  anterior  (Mormyrus). 
The  apparently  different  functions  of  these  two  similar  layers  are  then 
reversed.  The  striated  layer  is  sometimes  missing. 

These  three  layers  form  the  body  of  the  electroplax,  and  this  body  is 
best  understood  by  comparing  it  to  a  voluntary  muscle  fiber  which  has 
grown  wider  and  shorter  until  it  is  wider  than  it  is  long.  This  process  is 
carried  on  to  different  degrees ;  only  slightly  in  some  rays,  more  in  others, 

105 


io6 


HISTOLOGY 


and  in  Mormyrus  it  is  well  developed;  while  in  Torpedo  it  is  carried 
to  such  a  degree  that  the  electroplaxes  are  wide  and  thin;  so  thin  that 
layers  can  hardly  be  discerned.  The  broadening  and  development  of 
the  electroplax  takes  place  at  one  end  of  the  changing  muscle  fiber  in 
both  the  ontogeny  and  taxonomy  of  the  electric  tissues,  and  sometimes 
the  lower  end  of  the  electroblast  or  young  electroplax  remains  attached 
to  the  posterior  surface.  Figure  104  represents  a  series  of  diagrams  of 


FIG.  104.  —  A  -H.  A  series  of  diagrams  of  the  various  kinds  of  electroplaxes,  showing  in  part 
their  polarity  and  their  relations  to  the  striated  muscle  fiber.  All  nerve  structures  stippled. 
Striated  structures  indicated  by  lines.  A,  muscle  fiber;  B,  electroplax  of  young  Raja  batis; 
C,  electroplax  of  Raja  Icmis;  D,  electroplax  of  Tetronarce;  E,  electroplax  of  Astroscopus 
the  "stargazer";  F,  electroplax  of  Gymnotus,  the  electric  "eel";  G,  electroplax  of  Mor- 
myrus; H,  electroplax  of  Malapterurus.  Innervated  surface  above,  on  all  but  G. 

the  various  forms  of  electroplaxes,  showing  their  polarity  and  comparing 
them  with  a  striated  muscle  fiber. 

The  electroplax  is  developed,  with  the  possible  exception  of  Malap- 
terurus, from  an  electroblast  which  is  exactly  homologous  with  a  sarcoblast 
or  young  muscle  cell,  and  in  some  species  of  Raja  are  the  same,  as  far 
as  the  microscope  can  reveal.  Thus  we  see  that  the  electroplax  may  be 
considered  to  be  a  modified  muscle  fiber  whose  action  produces  elec- 
tricity instead  of  motion. 

The  distribution  of  nuclei  in  the  electroplax  is  characteristic;  they 
form  a  continuous  single  layer  in  the  electric  layer  and  posterior  layer 
and  are  separated  by  regular  intervals  from  each  other. 

Each  is  surrounded  by  a  bit  of  granular,  undifferentiated  cytoplasm, 
much  as  is  the  nucleus  of  a  smooth  muscle  cell  in  the  vertebrate  bladder 


ELECTKIC    7VSSUES  IO/ 

or  intestine.  Others  are  found  scattered  sparingly  through  the  middle 
layer.  The  middle  or  striated  layer  represents  the  striated  body  of  the 
striated  muscle  fiber  in  which  the  prominent  lamellae  of  muscle  units 
are  more  or  less  absent.  In  some  of  the  most  powerful  and  efficient 
electric  organs  this  middle  layer  is  apparently  missing,  leaving  only 
parts,  the  electric  network,  which  exists  between  the  lamellae  in  less 
specialized  forms  and  also  may  exist  in  normal  striated  muscle  cells. 
A  thin  membrane  surrounds  the  whole  syncytium  and  is  called  the 
electrolemma. 

The  specific  cell-organ  through  the  action  of  which  electricity  is  dis- 
charged is  probably  a  series  of  fine  rod-like  structures  attached  to  the 
electrolemma  and  directed  towards  the  anterior  layer  on  which  they  rest. 
They  have  not  been  actually  described  as  present  in  all  electric  tissues, 
but  are  probably  so  present.  They  are  exceedingly  small  and  in  some 
forms  are  straight  and  simple,  and  in  others  are  curved  and  provided 
with  peculiar  end-knobs,  and  sometimes  are  combined  into  groups  of 
two  or  three. 

These  electric  rods  are  sometimes  found  all  over  the  surface  of  the 
electrolemma ;  in  other  cases  they  occur  only  at,  or  near,  such  points  of 
it  as  are  touched  by  the  end-plates  of  the  nerve  that  supplies  it. 

While  the  electric  rods  are  probably  the  means  through  which  the 
electricity  is  discharged,  there  should  be  another  cell-organ  by  which  it 
is  stored  up  in  some  potential  form  through  nutritive  processes.  A 
protoplasmic  network  of  fine,  irregular  fibers  in  which  granules  are 
embedded  has  been  surmised,  in  Raja  and  other  forms,  to  be  such  an 
organ.  This  network  pervades  the  entire  electroplax  more  or  less, 
being  found  between  the  lamellae  only  of  the  striated  layer  when  these 
lamellae  are  present.  This  network  probably  acts  as  an  area  of  deposit 
for  the  numerous  granules  that  are  secreted  by  the  cytoplasm.  We  shall 
call  such  granules  the  electrochondria,  as  they  are  probably  homologous 
to  the  myochondria  of  the  muscle  cell,  and  used  to  produce  electricity 
by  some  chemical  process. 

The  nerve  supply  enters  the  compartment,  usually  from  an  anterior 
corner,  as  one  or  more  medullated  fibers  derived  from  the  electrit  nerve, 
which  is  a  modified  motor  nerve.  These  fibers  lose  their  medullary  sheath 
soon  after  entering  the  compartment,  and  branch  and  rebranch  until 
they  touch  the  electroplax  at  many  points  and  spread  out  into  the  end- 
plates.  These  nerve  end-plates  form  an  irregular  but  characteristic 
network  that  covers  most  of  the  electric  surface  of  the  electroplax.  This 
area  of  contact  between  nerve  and  electroplax  is  reduced  to  a  number 
of  evaginated  points  in  Gymnotus,  the  electric  eel;  to  still  fewer  in  Mor- 
myrus,  and  to  one  point  in  Malapterurus,  the  electric  catfish. 

The  rays,  Raja,  present  an  electroplax  in  which  the  general  features 


IO8  HISTOLOGY 

of  structure  can  probably  be  demonstrated  and  their  relations  understood 
as  well  as  in  any  form.  The  electroplax  is  not  highly  specialized,  and 
material  is  easily  obtained  all  over  the  world.  A  complete  demonstra- 
tion is  somewhat  delicate  and  difficult,  and  nitrate  of  silver  preparations 
are  essential  to  a  demonstration  of  the  fundamental  points. 

Technic.  —  It  is  more  true  of  electric  tissue  than  of  muscle  that  chance 
plays  a  large  part  in  the  winning  of  good  results.  Several  of  the  better 
fixatives  should  be  tried,  and  the  use  of  dead  tissue  which  has  recently 
died  of  secondary  causes  (as  bleeding  or  suffocation  or  narcotics)  should 
not  be  neglected.  Nitrate  of  silver  used  after  the  rapid  method  of  Golgi 
is  an  all- important  method  in  the  study  of  electric  tissue,  and  should  be 
used  in  the  study  of  each  form.  It  serves  to  demonstrate  much  more 
than  the  nerve  connection  of  the  electroplax. 

LITERATURE 

No  really  comprehensive  survey  of  the  subject  has  been  written  other  than  the  shorter 
accounts  in  the  text-books  of  zoology  and  some  encyclopaedias.  The  subject  must  be 
read  up  in  the  separate  articles,  some  of  which  are  mentioned  after  the  following  parts. 


ELECTRIC   TISSUE  OF   ELASMOBRANCH   FISHES 

The  electric  tissues  of  Raja  ocellata  consist  of  two  modifie  regions  of 
the  tail  muscle ;  a  symmetrical  spindle-shaped  area  in  each  of  the  mus- 
cular masses  that  lie,  one  on  each  side  of  the  tail.  The  spindles  begin 
anteriorly  in  this  ray  at  about  the  level  of  the  ventral  fins  and  extend 
almost  to  the  end  of  the  tail.  The  electric  tissue  can  be  easily  distin- 
guished from  the  surrounding  muscle  tissue  in  the  fresh  specimen  by  its 
jelly-like  appearance. 

The  organ  is  divided  into  minute  compartments  whose  outline  is 
apparent  on  the  outer  surfaces  as  well  as  on  cut  surfaces  of  the  spindle. 
The  dividing  walls  of  these  compartments  are  of  a  white  fibrous  connec- 
tive tissue,  and  the  interior  of  each  compartment  is  filled  with  a  jelly-like 
connective  tissue,  the  electric  connective  tissue  in  which  the  electroplax 
lies.  From  the  anterior,  inner  corner  or  edge  of  the  compartment,  a 
nerve  supply  enters  to  innervate  the  electroplax.  Blood  vessels  are 
introduced,  usually  from  the  opposite  or  posterior  side,  and  branch  in 
the  electric  connective  tissue  to  furnish  the  electroplax  with  blood. 

The  compartment  is  wider  than  it  is  long,  a  polyhedral  cavity  placed 
with  its  two  large  surfaces  cephalad  and  caudad.  It  will  average,  in  a 
three-foot  ocellata,  about  600  to  800  microns  in  width  and  390  to  450 
microns  in  length.  The  myotomes  of  the  tail  muscles  are  continued 
directly  into  the  electric  organ,  dividing  its  mass  of  electroplaxes  into  a 


ELECTRIC   TISSUES   OF  ELASMOBRANCHS 


109 


•nut. 


series  of  concentric  cones  that  may  be  called  the  electrotomes.  The  rela- 
tions of  these  electrotomes  to  the  myotomes  are  further  explained  in  the 
next  section  on  the  development  of  the  electric  organs  in  the  embryo 
skate. 

The  electroplax,  which  is  the  structure  that  produces  the  electricity, 
is  a  large,  disk- 
shaped  syncytium 
that  lies  within  the 
compartment  with  d.  1. — 
its  width  agreeing  elnur 
with  the  width  of 
the  cavity.  It  does 
not  occupy  the  en- 
tire length  of  the 
cavity,  however, 
but  leaves  an  an- 
terior and  a  pos- 
terior space.  These 
spaces  are  usually 
of  considerable  size 
and,  as  has  been 
indicated  before, 
are  filled  with  the 
electric  connective 
tissue  whose  in- 
dividual cells  are 
branching  forms, 
as  seen  in  Figure 
105.  These  cells 
secrete  the  jelly- 
substance  of  the 
tissue. 

The  spaces  vary 
much  in  different 
skates,  the  anterior 
being  many  times 
larger  than  the  posterior  in  a  Raja  ocellata,  while  in  Raja  lavis  the 
exact  opposite  is  true.  It  is  in  the  electric  connective  tissue  of  the  an- 
terior space  that  the  nerves  supplying  the  electroplax  ramify.  These 
nerves,  which  consist  of  several  medullated  fibers,  when  well  within  the 
compartment,  lose  their  medullary  sheaths  and  the  fibers  begin  to  divide 
and  sub-divide  as  they  approach  the  anterior  surface  of  the  electroplax 
on  which  their  ramifications  terminate  in  a  large  number  of  end-plates. 


FlG.  105.  —  Portion  of  an  electroplax  of  Raja  lavis.  el.c.t.,  electric 
connective  tissue  before  the  electroplax;  el.n.e.,  electric  nerve 
ending;  el. I.,  electric  layer;  el.nu.,  electric  nucleus;  $lr.,  striated 
region  of  electroplax;  nut. I.,  nutritive  layer.  The  mass  of  stri- 
ated material  in  the  large  central  papilla  is  cut  off  optically  from 
the  striated  layer  by  the  irregularity  of  the  papilla. 


HO  HISTOLOGY 

These  end-plates  appear,  in  a  section,  as  a  rather  closely  set  row  of  tiny, 
transparent  areas  lying  on  the  anterior  surface  of  the  electroplax  (Fig. 
105,  el.n.e.}.,  The  finer  non-medullated  ramifications  of  the  nerve  are 
closely  associated  with  the  connective-tissue  cells  whose  nuclei  are  to  be 
seen  lying  alongside  of  the  branches  of  the  fiber  at  many  points  near  the 
electroplax  (Fig.  105,  el.c.t.}. 

The  shape  of  the  electroplax  is  that  of  a  rather  thick  disk  with  its 
circular  edge  slightly  thinned  and  bent  upward  (anteriorly).  It  has, 
therefore,  been  called  saucer-shaped  or,  where  in  other  skates  the  bent 
edge  is  higher,  cup-shaped.  Its  shape  varies  much  in  the  different  spe- 
cies of  skates. 

The  electroplax  is  composed  of  three  layers,  the  two  outer  of  which  are 
continuous  around  the  edge  and  must  be  regarded  as  two  (an  anterior 
and  a  posterior)  specialized  areas  of  a  general  outer  layer.  The  middle 
or  inner  layer  is  of  a  very  different  structure  and  much  the  thickest.  It 
forms  a  core  or  inner  portion  and  is  striated.  These  striations  represent 
cross  sections  of  a  series  of  undulating  and  parallel  lamellae.  They 
correspond  to  similar  structures  in  striated  muscle,  and  under  the  highest 
powers  are  seen  to  be  formed  of  sheets  of  upright  rods. 

With  different  fixatives  and  in  different  specimens  the  appearance  of 
these  rods  varies,  much  as  does  that  of  the  muscle-rods.  At  times  a 
row  of  dots  forms  an  equidistant  line  between  them  and  again  the  rods 
themselves  appear  double  or  very  short,  with  two  dots  on  either  end.  A 
fine  transparent  fibril  can  be  seen  running  at  right  angles  to  the  lame  11  e 
and  connecting  the  rods  into  fibrillar  structures  like  those  of  muscle. 
This  fibril  probably  is  homologous  with  that  of  striated  muscle  tissue,  and 
the  striation  is  due  to  the  exact  and  equal  arrangement  of  the  anisotropic 
and  isotropic  substances  on  the  parallel  fibrils.  Their  very  small  size 
prevents  satisfactory  studies  of  their  structure. 

The  lamellae  are  very  much  denser  and  finer  in  Raja  lavis,  and  it  is, 
in  this  latter  species,  almost  impossible  to  see  the  rods  that  compose  an 
anisotropic  line.  Here,  too,  there  is  far  greater  density  and  continuity 
of  the  striation,  and  they  are  not  so  irregularly  arranged  as  in  ocellata. 
The  whole  syncytium  is  surrounded  by  a  delicate  cell-membrane,  the 
electrolemma,  which  corresponds  to  the  sarcolemma  in  a  voluntary  muscle 
cell. 

While  the  anterior  surface  is  flat  the  posterior  surface  is  evaginated 
into  many  papillae  that  vary  in  the  individual  electroplaxes  as  to  width 
and  length.  The  posterior  layer  covers  these  papillae  and  follows  all 
their  turns  and  bends.  Where  the  width  of  a  papilla  is  less  than  that 
of  a  double  thickness  of  the  posterior  layer,  no  striated  or  middle  layer 
exists  except  certain  broken  off  and  irregular  portions  of  striated  material 
lying  in  the  papillae.  In  Raja  l&vis  the  striated  substance  is  pushed 


ELECTRIC    TISSUES   OF  ELASMOBRANCHS  1 1 1 

farther  down  into  the  papillae,  and  larger  areas  are  isolated  from  these 
main  masses. 

The  nuclei  are  found  in  all  three  layers,  forming  a  very  regular,  close- 
set  arrangement  in  the  electric  layer,  very  sparingly  scattered  through  the 
striated  region,  and  numerous,  but  irregularly  arranged,  in  the  nutritive 
layer.  The  nuclei  of  this  last  region  seem  somewhat  smaller,  more 
irregular  in  outline,  and  with  denser  chromatin  masses  than  those 
found  in  the  other  layers.  Each  is  surrounded  by  a  mass  of  granu- 
lar cytoplasm.  These  masses  are  connected  and  form  a  separate 
layer. 

The  above  section  that  we  have  been  studying  is  a  longitudinal, 
vertical  section  of  the  spindle,  and  consequently  a  longitudinal  section 
of  the  electro plax.  The  material  was  fixed  in  Flemming's  strong  fixative 
and  stained  in  iron-haematoxylin.  In  order  to  proceed  farther  with  an 
understanding  of  the  tissue  it  will  be  necessary  to  prepare  other  fresh 
material  with  Golgi's  quick  silver  method  and  to  cut  sections  at  right 
angles  to  the  anterio-posterior  axis  of  the  electroplax.  The  sections  will 
be  parallel  to  the  neuro-electric  surface,  the  flat  surface  on  which  the 
nerve  terminates  in  many  fine  branches.  When  such  sections  are 
examined  as  include  all  or  any  portion  of  this  surface,  we  can  distinguish, 
in  favorably  stained  parts  of  the  specimen,  the  following  facts :  — 

The  ground  substance  of  the  electric  layer  forms  a  coarse  network, 
whose  regular  rounded  meshes  contain  the  nuclei.  Each  nucleus  is  sur- 
rounded by  its  own  portion  of  unspecialized  cytoplasm,  and  a  thin  layer 
of  the  ground-substance  covers  the  top  and  bottom  of  each  mesh  with  a 
delicate  layer.  Thus  each  nucleus,  with  the  unspecialized  cytoplasm 
that  surrounds  it,  is  inclosed  in  an  oval  space  entirely  within  the  electric 
layer. 

The  ground-substance  shows  a  distinct  reticulum  or  network  of 
fibrils.  This  network  is  very  fine  and  dense  and  stains  brown-black. 
Its  fibrils  are  irregular  and  granular,  and  at  times  can  be  seen  to  run  some 
considerable  distance  without  anastomosing.  The  reticulum  is  coarser 
in  some  regions  than  in  others,  and  this  is  so  marked  that  we  may  dis- 
tinguish between  a  fine  and  a  coarse  reticulum. 

How  far  does  this  reticulum  extend  into  the  substance  of  the  elec- 
troplax and  what  relation  has  it  to  the  striations?  A  cross  section  of 
the  silver  material  will  show  this,  and  in  it  we  see  that  it  pervades  the 
ground-substance  and  is  found  between  the  striations  or  lamellae  only. 
Thus  it  is  not  directly  continuous  in  the  electroplax. 

A  remarkable  feature  that  is  revealed  by  the  silver  process  is  the  mul- 
titude of  tiny  pointed  rods  projecting  from  the  inner  surface  of  the  elec- 
trolemma  into  the  electric  layer  for  a  distance  of  several  microns.  These 
structures  are  very  dense  and  refractive  and  are  only  found  on  that  part 


112 


HISTOLOGY 


of  the  electric  surface  that  is  in  contact  with  the  ramifications  of  the  nerve- 

ending.    They  are  sharp,  wedge-shaped,  and  very  small  (Fig.  106). 
A  brief  examination  of  the  electroplax  of  the  "torpedo  fish,"  Tetro- 

narce,  should  be  made  at  this  point  to  compare  its  highly  specialized 

electric  unit  with  the  simpler  and  more  rudimentary  structure  of  the 

electroplax  found  in  Raja. 

In  Tetronarce  the  electric  organ  is  composed  of  two  masses  of  vertical 

columns,  each  column  with  six  sides  to  fit  closely  with  its  neighbors. 
Each  column  is  a  pile  of  very  thin  electro- 
plaxes  that  lie  with  their  two  surfaces  at  right 
angles  to  the  column  and  occupy  its  entire 
section,  excepting  that  their  corners  are  some- 
what rounded.  A  portion  of  electric  con- 
nective tissue  lies  on  each  side  of  the  electro- 
plax and  they  are  farther  separated  by  a  very 
thin  layer  of  white  fibrous  tissue.  The  columns 
are  bounded  on  their  sides  by  comparatively 
heavy  walls  of  the  white  fibrous  connective 
tissue. 

Examination  of  a  vertical  section  of  a  part 
of  the  column  shows  the  electroplaxes  in  cross 
section.  They  are  so  thin  when  seen  thus 
that  it  is  with  great  difficulty  that  they  are 
made  to  appear  as  more  than  a  line  with 
several  nuclei  on  it.  The  nuclei  are  clearly 

FIG.  106.  —  Part  of  the  nerve-    made  out  to  be  of  two  kinds:     a  large  round 

£"££  IT--  one  on  the  upp.er  side  and  really  b^ng  in  the 

Nerve-ending  seen  be-    electroplax,  which  is  thinner  than  the  diameter 

of  the  nucleus;  also  a  smaller  and  somewhat 

elongated  nucleus  of  denser  appearance,  which 
is  clearly  a  connective-  tissue  element.  This  is  usually  lying  on  or  near 
the  electric  surface  of  the  electroplax.  A  fairly  well  fixed  specimen 
of  this  tissue  shows  two  visible  layers  of  the  structure. 

Surface  studies  of  this  tissue  with  the  Golgi  method  show,  as  in  Raja, 
the  electric  end-organ  of  the  nerve  supply  as  well  as  the  numerous  small 
rods  that  point  down  into  the  electric  layer  and  are  found  on  the  elec- 
trolemma  only  at  such  of  its  points  as  are  in  contact  with  the  nerve  end- 
organ.  This  latter  structure  is  as  complicated  as  in  the  skate.  Each  of 
the  rods,  instead  of  being  a  simple  pointed  wedge  as  in  the  skate,  is  of 
stouter  formation,  and  bears  on  its  end  a  peculiar  round  knob.  Some- 
times two  of  these  rods  are  united. 

The  Golgi  method  also  shows  a  reticulum  in  the  cytoplasm  that  is 
similar  to  that  seen  in  the  skate.  In  general,  the  electroplax  of  Tet- 


fiber. 


HISTOGENESIS   OF  ELECTRIC    TISSUES  113 

ronarce  is  similar  to  an  electroplax  of  Raja  which  has  become  wider 
and  very  much  thinner,  so  thin  that  the  striated  layer  is  obliterated  and 
the  others  reduced  to  a  minimum. 

LITERATURE 

ENGELMANN.     "  Die  Blattschicht,  etc.,  der  gew.  Rochen,"  Arch.  f.  Physiol.,  Pfluger,  Band 

LVII,  t.  2,  S.  149. 
BALLOWITZ.    "liber  den  feineren  Bau  des  Elektrischen  Organs  des  Gewohnlichen  Rochen," 

Arch.f.  mik.  Anat.,  Band  XLII,  1892. 


ONTOGENETIC  DEVELOPMENT  OF  THE  ELECTROPLAX   IN  ELAS- 
MOBRANCH   FISHES 

A  study  of  the  developing  electric  organ  in  a  skate  is  most  illuminat- 
ing as  to  the  real  significance  of  this  tissue.  It  has  been  worked  out  in 
a  common  form  of  skate,  Raja  batis,  by  Dr. 
J.  C.  Ewart,  and  the  following  description  is 
drawn  from  that  paper  and  the  paper  by 
Englemann. 

The  young  embryo  of  this  skate  has  no 
indication  of  any  electric  tissue.  The  place 
that  will  be  occupied  by  this  organ,  a  little 
later  in  the  development,  is  filled  by  the 
young  muscle  fibers  that  differ  in  no 
visible  way  from  those  about  them  (Fig. 
107,  A). 

In  an  embryo  of  about  7  cm.  the  first 
appearance  of  the  development  of  electric 
tissue  is  a  swelling  of  the  anterior  ends  of 
the  muscle  fibers  in  the  centers  of  the  future 
electric  spindles  (Fig.  107,  B}.  The  nuclei 
have  increased  in  number  in  this  enlarged 
part  of  the  fiber,  and  some  of  them  appar- 
ently have  migrated  from  among  the  muscle 
fibrils  and  come  to  lie  in  the  undifferentiated 
cytoplasm  between  the  fibril-bundle  and  the 
sarcolemma  (by  sarcolemma  in  this  case  is 
meant  a  cell-membrane  which  is  somewhat 
more  evident  in  young  muscle  cells  and  in 
some  electroplaxes  than  in  most  older 
muscle  fibers).  These  changes  first  occur  in 
the  fibers  that  occupy  the  central  axis  of  the  future  spindle-shaped  organ 
and  then  in  successive  outer  shells  of  the  fibers  until  the  organ  is  com- 
pleted. All  future  changes  occur  in  the  same  order. 


FIG.  107.  —  A-D.  Four*  stages  in 
the  development  of  an  electro- 
plax from  a  muscle-like  electro- 
blast  in  Raja  batis.  A  is  in  all 
respects  like  a  muscle  fiber;  B 
shows  an  enlargement  of  the  an- 
terior end;  C  and  D  show  the 
steps  which  practically  complete 
the  process.  (After  J.  EWART.) 


114  HISTOLOGY 

In  an  embryo  of  about  10  or  n  cm.  in  length,  enough  variety  and  ad- 
vancement in  the  development  of  the  electroplax  can  be  found  to  supply 
all  needed  steps.  The  muscle  fiber  is  seen  here  with  the  anterior  enlarge- 
ment much  wider  and  heavier  and  showing  the  form  and  structure  of 
the  completed  electroplax  (Fig.  107,  C).  The  posterior  end,  on  the  other 
hand,  is  arrested  in  its  growth  and  development  and  in  the  older  cells  is 
actually  shriveled  into  a  ribbon-like  form  that  still  clings  to  the  electro- 
plax (Fig.  107,  D).  It  retains  its  striated  fibrils  for  a  time,  but  they  are 
gradually  absorbed  until  nearly  lost.  This  muscle-cell  remnant  is 
entirely  missing  in  most  other  skates. 

The  further  following  changes  have  also  taken  place  in  the  somewhat 
older  stages.  The  motor  nerve-ending  of  the  young  muscle  fiber  has 
moved  to  the  anterior  end  of  the  developing  electroplax  and  has  devel- 
oped to  form  the  electric  nerve-ending  which  lies  at  this  time  in  a  saucer- 
shaped  depression  on  the  end  of  the  structure.  The  striated  portion  of 
the  muscle  fiber  has  widened  and  shortened  to  form  the  striated  layer  of 
the  electroplax,  meanwhile  changing  its  comparatively  wide  and  straight 
bands  of  anisotropic  substance  to  narrower  and  curved  bands.  These 
striations  of  the  electroplax  are  still,  however,  as  strictly  parallel  as  were 
the  muscle  striations.  The  changes  undergone  by  the  striation  have  not, 
as  yet,  been  properly  investigated. 

The  electric  layer  has  been  formed  from  a  layer  of  the  muscle-nuclei 
lying  in  the  undifferentiated  cytoplasm  of  the  electroplax,  and  this  electric 
layer  has  extended  over  the  edges  of  the  electroplax  to  become  the  nu- 
tritive layer  on  the  posterior  side.  This  nutritive  layer  has  become  evagi- 
nated  into  a  number  of  papillae  of  considerable  length,  extending  into  the 
electric  connective  tissue  and  anastomosing  with  one  another  to  a  con- 
siderable extent. 

So  we  see,  in  this  development,  the  visible  and  undoubted  steps  of 
the  change  of  a  muscle  fiber  into  an  electroplax ;  thus  abundantly  cor- 
roborating the  surmises  that  this  was  the  case  in  the  electric  tissues  of 
Mormyrus  and  the  other  forms,  excepting  only  Malaptemrus. 

The  development  or  histogenesis  of  the  electroplax  in  Torpedo  can 
again  be  compared  with  the  process  in  Raja  to  great  advantage.  The 
organ  begins  in  this  latter  form  as  a  set  of  vertical  bundles  of  long  thread- 
like cells  each  with  one  nucleus  (Fig.  108,  A*).  These  cells  acquire  a  faint 
striation  as  does  a  young  muscle  cell,  and  then  the  lower  end  begins  to 
enlarge,  and  the  nucleus  which  is  near  the  lower  end  begins  to  divide 
(amitotically  probably)  into  the  large  number  of  nuclei  that  are  after- 
ward found  in  the  electroplax  (Fig.  108,  B).  The  club-shaped  end 
rapidly  widens  into  the  plate-like  electroptax,  while  the  upper  end  of  the 
fiber  atrophies  and  is  seen  no  more.  The  striation  persists  for  but  a 
short  time  in  its  ventral  edge  and  then  disappears  also.  The  striation 


ELECTRIC   TISSUES,    TELEOSTS  115 

seen  in  the  latter  parts  of  the  development  is  the  longitudinal  striation  or 
fibrillation  (Fig.  108,  C).  Earlier  in 
the  process  the  cross  striation  is  the 
most  prominent.  Each  bundle  of 
fibers  develops  into  one  of  the  col- 
umns of  the  completed  electric  or- 
gan. In  changing  its  longitudinally 
placed  sarcoblasts  (they  might  more 
properly  be  called  electroblasts)  into 
electroplaxes,  they  are  changed 
from  the  vertically  elongated  form 
into  a  horizontally  elongated  form 
without  changing  in  the  least  their 
morphological  position  or  axes.  The 
selachian  fishes  thus  show  a  com- 
plete homology  in  their  electro- 
plaxes. Unfortunately  we  do  not 
know  the  embryology  of  the  more 
numerous  teleostean  forms. 

Technic. — The  use  of  nitrate  of 
silver  by  the  Golgi  method  has  been 
carried  even  into  the  embryonic 
tissue,  although  without  the  decided 
results  that  it  has  yielded  in  the 
adult  tissue.  Methylene  blue  has 

not  been  successfully  used  in  any  electric  tissue  with  the  exception  of 
Raja  by  Retzius,  who,  however,  has  neglected  to  announce  the  secret 
of  his  success,  if  there  be  any  secret  that  can  be  stated. 

LITERATURE 

OGNEFF,   J.     "Uber  die  Entwicklung  des  electrisches  Organs  bei  Torpedo,"  Arch.  f. 

Anat.  u.  Physiol.,  1897,  S.  270. 
EWART,  J.  C.     "Development  of  the  Electric  Organs  in  the  Skate,"  Phil.  Trans.  Roy. 

Soc.,  Vol.  CLXXIX  B,  p.  399. 
ENGELMANN,  TH.  W.    "Die  Blatterschicht  der  Elek.  Organe  von  Raja  in  ihren  gertetischen 

Beziehungen  zur   quergestriften   Muskelsubstanz,"   Pfluger's  Archiv,   Band   LVII, 

1894,  S.  149- 


FIG.  108.  — A-C.  Three  steps  in  the  histo- 
genesis  of  an  electroplax  of  Tetronarce,  the 
"torpedo"  or  "  numbfish."  The  body  of 
the  electroplax  is  derived  from  the  pos- 
terior end  of  the  fiber  instead  of  from  the 
anterior,  as  in  Raja.  (After  OGNEFF.)  The 
weak  striae  represent  the  fibrillation  rather 
than  the  cross  striation  of  muscle. 


THE   ELECTROPLAX  IN   TELEOST   FISHES 

The  teleest  fishes  present  a  number  of  examples  of  electric  tissues 
which  are,  with  one  exception,  recognized  to  be  modified  muscular  tissue 
upon  anatomical  and  histological  grounds.  Unfortunately  the  embry- 
ology and  histogenesis  of  these  organs  has  not  been  investigated  for 
lack  of  material.  This  is  especially  unfortunate  in  the  case  of  Malap- 


Il6 


HISTOLOGY 


terurus,  whose  electroplaxes  are  placed  in  the  skin  and  are  consequently 
in  doubt  as  to  their  origin. 

We  shall  briefly  describe  the  electroplax  as  found  in  three  genera 
of  teleost  fishes,  Gymnotus,  Mormyrus,  and  Astroscopus,  making  some 

comparisons  and 
afterward  discuss- 
ing briefly  the  pe- 
culiar electric  organ 
7T.  's  of  Malapterurus. 

The  electric  tis- 
sue of  Gymnotus, 
which  is  an  eel- 
shaped  fish  inhab- 
iting some  fresh 
waters  of  South 
America,  consists 
of  a  number  of 
rows  of  electro- 
plaxes placed  verti- 
cally and  face  for- 
ward, to  form 
several  masses  of 
tissue  on  the  sides 
of  the  rear  part  of 
the  body. 

This  tissue  oc- 
cupies the  place 
ordinarily  filled  by 
muscle,  and  this, 
together  with  the 
fact  that  the  elec- 
troplaxes are  placed 
in  a  myotome  ar- 
rangement, forms 
fairly  good  evidence  that  the  tissue  is  derived  from  muscle  rudiments. 
When  we  add  to  this  that  we  know  the  electric  tissues  to  be  modified 
muscle  in  the  elasmobranchs  where  it  occurs,  we  have  a  strong  con- 
firmation of  its  relationship. 

Each  electroplax  is  placed  with  its  electric  surface,  on  which  the  nerve 
supply  ends,  facing  directly  to  the  rear,  and  the  other  side  or  nutritive 
surface  facing  forward.  Both  in  front  of  and  behind  the  plate  is  a  layer 
of  the  electric  connective  tissue  or  jelly,  the  one  in  front  being  somewhat 
the  thinner. 


FlG.  109.  —  Portion  of  an  electroplax  from  Gymnotus,  the  "  electric 
eel."  el.s.,  electric  surface;  nu.s.,  nutritive  surface;  el.c.t.,  elec- 
trie  connective  tissue.  Papillae  on  both  surfaces.  (After  BAL- 

LOWITZ.) 


ELECTRIC   TISSUES,    TELEOSTS 


117 


The  posterior  surface  is  evaginated  into  a  large  number  of  short, 
medium  thick  papillae,  while  the  anterior  face  is  drawn  out  into  very 
many  very  thick  projections,  so  thick  and  so  closely  set  that  their  sides 
touch,  for  the  most  part.  Each  papilla  system  may  roughly  be  said  to 
be  as  thick  as  the  solid  middle  layer. 

Figure  109  shows  a  small  part  of  a  section  taken  transversely  to  the 
electroplax.  The  cytoplasm  is  seen  to  be  a  delicate  reticular  substance 
slightly  denser,  perhaps,  toward  the  anterior  surface  and  especially  around 
the  edges  of  the  spaces  which  separate  the 
anterior  papillae. 

The  edges  of  the  section  show  that  over 
the  entire  surface  of  the  electroplax  there  is  a 
continuous  series  of  short  rod-like  structures 
pointing  at  right  angles  from  the  electrolemma 
into  the  cytoplasm.  Those  on  the  posterior 
surface  are  the  heaviest,  and  may  represent 
the  "  stabchen "  or  little  rods  first  described 
by  Ballowitz  in  the  torpedo. 

The  nuclei  are  not  numerous  when  one 
considers  the  large  mass  of  cytoplasm  that 
they  must  care  for.  They  are  found  near 
the  edge  and  usually  out  in  the  papillae,  more 
in  the  posterior  than  the  anterior. 

The  nerve  fibers  approach  the  electroplax  FlG  II0._Endsof 
in  the    posterior  jelly  layer    as   medullated      from  the  electric  surface  of 
fibers,  which  divide  and  send  non-medullated      ^etSnVng  %££* 

branches  to  the  ends  Of  the  posterior  Or  elec-        mesh-shaped  nerve-endings  on 

trie  papillae.     Here  the  nerve  forms  its  motor 

endings,  consisting  of  a  thick,  heavy  plexus 

that  embraces  the  upper  part  of  the  papilla  lying  in  an  intimate  contact 

with  its  cytoplasm.      Figure  no  shows  a  surface  view  of  the  ends  of 

two  of  the  papillae  with  their  relations  to  the  nerve  supply. 

Comparing  this  organ  with  the  electroplax  of  Raja,  which  we  hold  to 
be  the  most  rudimentary  and  complete  electroplax,  it  can  be  seen  that 
it  is  highly  differentiated.  The  striation  is  absent,  which  shows  special- 
ization, and  the  points  of  contact  with  the  ends  of  the  nerve  fibers  are 
multiple,  which  seems  to  argue  for  a  multicellular  origin  for  the  electro- 
plax. 

The  electric  tissue  of  Mormyrus,  as  described  by  Schlichter,  shows 
some  striking  similarities  to  that  of  Gymnotus,  as  well  as  some  wide 
differences.  As  in  the  electric  eel  (it  is  not  an  eel,  but  another  kind  of 
elongate  fish)  the  tissue  is  a  modified  portion  of  the  posterior,  lateral, 
body  musculature  and  is  also  composed  of  upright  plates  or  electroplaxes 


the    papillae.     (After    BALLO- 
WITZ.) 


HISTOLOGY 


facing  directly  forward  and  backward.  The  anterior  face  is  smooth  and 
is  the  nutritive  surface.  The  posterior  or  electric  surface  is  regular  and 
sends  out  a  number  (few  compared  to  those  of  Gymnotus]  of  evaginated 
processes  that  are  long  and  usually  curved.  The  electric  nerve  applies 
its  curious,  heavy  mass  of  endings  to  these  processes,  and  in  the  ordi- 
nary preparations  one 
has  difficulty  to  say 
when  the  nerve  ends 
and  the  process  begins. 
The  nuclei  are 
fairly  numerous 
throughout  the  cyto- 
plasm and  are  large 
and  clear.  The  elec- 
tric rods,  as  in  Gym- 
notus, are  found  in 
all  surfaces,  even  those 
of  the  processes,  and 
they  are  longer  and 
sharper  than  in  that 
form.  In  some  prepa- 
rations the  elect ro- 
chondria  are  very 
clearly  seen  and  are 
large  granules  resting 
among  the  rods  (Fig. 
in). 

FIG.  in.— Portion  of  an  electroplax  from  Mormyrus.   el.s.,  elec-  The   most  peculiar 

trie  surface  from  which  part  of  a  papilla  projects;    mus.f.,  feature  of  this  electro- 
muscle  fibers  in   middle  layer  of  electroplax.     Electric  rods  ,  ,         . 

shown  on  both  surfaces.     (After  SCHNEIDER.)  plax  IS  the  tact  that  it 

contains,  as  a  middle 

layer,  a  series  of  small  but  perfect  muscle  bundles,  which  run  in 
several  directions  in  the  plane  of  the  electroplax.  These  are  each  com- 
posed of  a  number  of  real  myo-fibrils  fully  striated,  but  in  a  somewhat 
different  pattern  from  the  striation  of  the  regular  body  muscle. 

In  this  form  again  we  have  the  same  two  indications  of  a  multicellular 
origin  of  the  electroplax.  Besides,  and  added  to  these,  is  the  presence 
of  the  myo-fibril  bundles,  each  of  which  would  seem  to  represent  the 
functionless  remains  of  one  of  the  constituent  fibers  that  helped  form  the 
plate. 

The  last  of  these  teleost  forms  of  electric  tissue  is  found  in  the  fish, 
Astroscopus,  which  has  developed  what  appears  to  have  been  part  of 
an  eye-muscle  into  its  electric  organ.  This  tissue  is  again  composed 


ELECTRIC   TISSUES,    TELEOSTS 


119 


of  flat  plates  or  electroplaxes  (Fig.  112).  The  parallel  plates  lie  hori- 
zontally in  the  fish's  head,  and  the  upper  or  electric  surface  of  each 
electroplax  is  flat  and  smooth  and  receives  the  nerve-endings,  which  are 
somewhat  like  those  of  Raja  and  Torpedo  in  form  (Fig.  113). 

The  lower  or  nutritive  layer  is  evaginated  into  a  large  number  of  long 
papillae,  which  anastomose  somewhat  and  project  downward  from  the 
plate  for  about  twice  its  thickness. 

The  nuclei  are  numerous  and  are  differentiated  into  two  groups,  the 


FIG.  112.  —  Portion  of  a  vertical  section  through 
the  electroplax  of  Astroscopus.  el.l.,  electric 
layer  containing  the  electric  nuclei  and  the 
peculiar  fibers  or  rods. 


FIG.  113.  —  Silver  nitrate  picture  of  a  nerve- 
ending  on  electric  surface  of  the  electroplax 
in  Astroscopus.  wv.f.,  nerve  fiber.  Nerve- 
ending  is  black.  The  reticular  tissue  of  the 
electric  layer  shows  regularly  arranged 
spaces  in  which  lie  the  transparent  electric 
nuclei. 


electric  group  lying  in  the  uppermost  of  the  three  very  faintly  defined 
layers  into  which  the  plate  may  be  divided.  These  nuclei  are  equally 
spaced  in  this  layer  and  appear  very  regular  in  position.  The  second 
set  are  those  few  remaining  ones  which  are  scattered  through  the  lower 
part  of  the  electroplax,  principally  in  the  papillae.  The  rods  have  not  yet 
been  certainly  demonstrated  in  this  form. 

Besides  the  points  described  above,  Astroscopus  has  two  peculiar 
features.  The  entire  cytoplasm  of  the  electroplax  is  striated  uniformly 
with  a  series  of  fine,  close-set  striae  that  run,  as  in  Raja,  in  curved  parallel 
groups.  This  striation  is  probably  a  vestigial  indication  of  the  muscle 
tissue  from  which  the  electroplax  was  developed. 


120 


HISTOLOGY 


The  other  feature  appears  to  be  unique,  and  consists  of  a  series  of 
peculiar  pointed  fibers  and  long,  pointed  rods  lying  in  the  cytoplasm  of 
the  electric  layer.    The  use  of  these  structures,  which  are  shaped  like  the 
classic  "  thunderbolts,"  is  unknown.    They  might  pos- 
sibly be  elongated  "  rods  "  such  as  are  found   in  the 
other  forms.      Figure    112,  from   doubtful   material, 
shows  a  possible  thin  nutritive  layer  which  may  be 
an  artifact. 

Malapterurus  has  developed  its  electric  cell  in  that 
part  of  its  integument  which  surrounds  the  middle 
region  of  the  body.  The  large,  round,  flat  electroplaxes 
occupy  a  vertical  position  facing  forward  and  backward, 
as  in  Gymnotus  and  Mormyrus.  The  surface  of  one 
of  these  plates  is  moderately  regular,  with  the  excep- 
tion of  the  single  large  evaginated  process  which 
reaches  backward  from  an.  anteriorly  bent  and  cup- 
shaped  area  of  the  middle  of  the  plate.  This  process, 
which  is  as  long  as  a  quarter  of  the  diameter  of  the 
plate,  is  met  by  the  motor  nerve,  which  ends  in  its 
FIG.  114.  -Ending  extremity,  as  a  curled,  rod- 
of  electro-motor  like,  motor  end-organ  (Fig. 

SlKZS*    "4)-     The    electric     surfare 
process  of  an  eiec-   thus  faces  posteriorly. 

The  cytoplasm  Qf  the  elec. 

.  ,.    ,    ,  -^ 

troplax  is  very  lightly  reticular 

and  somewhat  granular,  and 
the  large  nuclei  are  much  less  numerous  than 
in  most  other  forms  of  the  organ.     The  edge     |g 
of   the  plate    shows,    in  transection,  a    layer     f-g 
similar  to  that  seen  in  Mormyrus,  and  almost     jpj 
equally  distributed  on  both  surfaces.     A  few        •».„; 
granules  of  peculiar  quality  with  some  very    (      °o     r  .:, 
few  coarse  fibrils  can  be  seen  in  the  cytoplasm 
near  the  nuclei  (Fig.  115). 

This  electroplax  has  been  though*  to  be  a 
development  of  a  gland  cell  in  the  skin  of  the 
fish.  The  writers  cannot  agree  with  this,  and 
consider  it  to  be,  more  probably,  a  specialized 
smooth  muscle  cell  of  the  dermis,  or  even  derived  from  a  layer  of  the 
striated  body  musculature.  The  embryology  and  histogenesis  of  this 
electric  organ  should  be  made  the  subject  of  investigation  at  the 
earliest  opportunity. 

Technic.  —The  same  as  for  the  other  electric  tissues.    No  embryo- 


troplax  of   Malap. 
terurus.  (After 

BALLOWITZ.) 


Malapterurus.   Rods  shown  on 
]£*  surfaces.    (After  BALLO- 


ELECTRIC    TISSUES,    TELE  OS  TS  121 

logical  tissues  have  ever  been  worked  on  in  the  case  of  the  teleost 
fishes. 

LITERATURE 

SCHLICHTER,  HEiNRiCH.     "  Uber  den  feineren  Bau  des  schwach-electrischen  Organs  von 

Mormyrus,"  Zeit.  f.  Wiss.  Zool.,  Band  LXXXIV,  S.  479,  1906. 
BALLOWITZ,  E.   "Zur  Anatomic  des  Zitteraales,"  Arch.  f.  mik.  Anat.,  Band  L,  Heft.  4, 

S.  686-750,  1897. 
BALLOWITZ,  E.     "Das  elektrische  Organ   des  Afrikanischen  Zitterwelses,"  Jena,   1899, 

Verlag  von  Gustav  Fischer. 
DAHLGREN  AND  SILVESTER.      "The    Electric  organ  of     the    Stargazer,   Astroscopus," 

Anat.  Am.,  Band  XXIX,  S.  387,  1906. 


CHAPTER    X 
TISSUES   OF  PHOTOGENESIS   OR   LIGHT-PRODUCTION 

PROTOPLASM  can  produce  not  only  heat,  motion,  and  electricity,  as  was 
seen  in  the  preceding  pages,  but  it  can  produce  light  as  well.  This  power 
is  found  in  a  limited  number  of  organisms  that  are  somewhat  more 
numerous  and  much  more  widely  distributed  in  the  animal  kingdom 
than  are  creatures  that  produce  electricity. 

The  power  is  probably  a  specialization  of  the  same  or  similar  pro- 
cesses to  those  that  produce  heat,  motion,  and  electricity.  Briefly,  it 
consists  of  the  production  of  a  material  that,  when  exposed  to  the  action 
of  oxygen,  or  possibly  some  other  substance,  rapidly  unites  with  it,  and 
in  doing  so  gives  rise  to  light  waves. 

The  light  that  they  produce  may  differ  in  color  and  quantity,  accord- 
ing to  the  animal  that  produces  it,  from  a  green  light  to  various  shades 
of  red,  purple,  and  violet.  In  some  forms,  several  colors  or  shades  are 
emitted  by  the  same  organ  at  different  times  or  by  different  organs  on  the 
same  animal.  It  is  possible  that  some  creatures  give  out  a  light  that  is 
not  visible  to  the  human  eye,  although  it  may  stimulate  the  eyes 
of  other  forms  whose  eyes  are  adapted  to  perceive  it. 

The  substance  that  thus  gives  light  upon  oxidization  is  of  unknown 
chemical  formula.  Du  Bois  has  extracted  it  from  the  tissue,  probably 
in  a  very  impure  state,  and  has  made  it  produce  the  light.  He  has 
applied  the  name  luciferase  to  it  and  maintains  that  it  is  not  with  oxygen 
that  it  must  combine  to  produce  the  light,  but  with  another  substance 
in  the  blood,  to  which  he  has  applied  the  name  luciferine.  His  theory 
is  not  considered  proven,  and  oxygen  is  probably  the  reducing  agent. 
This  oxygen  may  be  brought  to  the  scene  of  action  in  some  unstable 
compound,  as  haemoglobin,  for  a  carrier.  Phosphorus  forms  no  im- 
portant part  of  the  luciferase,  as  we  shall  call  the  light-giving  secretion, 
using  one  of  Du  Bois's  names  without  accepting  his  theories. 

Luciferase  is  a  secretion,  the  product  of  protoplasmic  activity  in  chang- 
ing the  food  materials  brought  to  it  into  some  specific  substance  of  use 
to  the  organism.  It  can  be  seen  in  sections  and  teased  cells,  as  a  collec- 
tion of  granules  that  stain  very  readily  and  retain  the  stain  with  great 
tenacity.  Sometimes  it  remains  within  the  cell  and  is  used  in  situ,  the 
oxygen  being  brought  to  it.  At  other  times  it  is  discharged  from  the 


TISSUES   OF  LIGHT-PRODUCTION 


123 


cell  to  be  oxidized  by  materials  brought  to  it  or  by  the  free  oxygen  in 
air  and  water.  Wherever  used,  it  only  acts  properly  in  an  alkaline 
medium. 

The  cells  that  secrete  luciferase  are  of  many  kinds.  They  may  be 
ectodermal  or  mesodermal  in  origin.  In  a  few  cases  they  are  cells 
that  are  used  mainly  for  other  purposes,  and  the  light-production  is  a. 
secondary  matter.  Examples  of  such  are  the  muscle  cells  of  Ophiura 
and  the  blood  cells  of  Hystrix.  In  the  majority  of  cases,  and  always 
where  the  organ  is  of  high  efficiency,  the  cells  are  devoted  exclusively 
to  this  function,  although  it  can  easily  be  seen  that  they  originated  from 
some  of  the  ordinary  tissues  of  the  body,  as  the  fat  body  in  insects,  the 
epithelial  cells  in  fishes,  etc. 

Cells  modified  to  make  light  are,  in  the  primitive  forms,  epithelial  in 
character.  They  appear  as  low  or  tall  columnar  cells.  In  the  more 
highly  specialized  forms,  and  especially  in  the  forms  of  mesodermal  origin, 
they  are  grouped  into  a  mass  of  polygonal  cells.  Where  the  cells  are 
columnar  in  shape  the  nucleus  is  found  near  the  proximal  end,  as  in  other 
cases  of  columnar  gland  cells,  and  the  materials  are  absorbed  through 
the  proximal  surface  and  passed  in  the  process  of  elaboration  toward  the 
distal  end,  where  they  are  either  used  to  produce  light  in  situ  or  dis- 
charged to  be  used  outside  the  cell.  On  the  other  hand,  where  the  cells 
are  of  the  second  or  polygonal  shape,  the  food  materials  are  absorbed 
from  all  surfaces  and  the  luciferase  is  used  in  situ,  the  products  of 
combustion  being  passed  out  through  the  same  surfaces  into  the  blood. 

Sometimes  these  cells  are  found  alone  in  an  organism,  but  in  the 
majority  of  cases  and  almost  always  in  the  highly  developed  organs,  they 
are  accompanied  by  one,  two,  or  three  other  and  accessory  tissues;  the 
reflecting  tissues,  the  lens  tissues,  and  the  pigment  mantles. 

The  reflecting  tissues  are  of  two  forms :  the  connective-tissue  reflectors 
and  the  urate  reflectors.  The  first  of  these  is  a  very  ordinary  looking 
fibrous  or  thin-layered  connective  tissue  that  cannot  be  told  in  any  way 
from  a  common  lax  or  fibrous  connective  tissue  except  that  it  will  reflect 
the  light  most  perfectly.  It  is  developed  from  the  same  tissue  that 
forms  the  corium  of  the  skin,  and  its  nuclei  are  like  those  of  other  tissues 
of  the  same  kind.  Its  fibrils  or  plates  are  usually  developed  at  right  angles 
to  the  direction  of  light  emission.  As  far  as  can  be  seen,  the  refractive 
power  is  due  to  the  presence  of  innumerable  and  almost  invisible  particles 
deposited  in  the  substance  of  the  reflecting  tissue.  No  other  tissue  in 
the  body  can  reflect  the  light  in  this  way  except  the  pigment  found  on  the 
surface  of  the  body  in  some  fishes. 

The  second  kind  of  reflector  is  made  of  layers  of  large  cubical  cells 
that  have  deposited  in  their  interior,  crystals  of  some  urate  that  reflects 
the  light  most  perfectly.  This  form  is  found  in  the  insects  and  perhaps 


124  HISTOLOGY 

in  the  crustaceans  with  light  organs.  It  is  fully  as  efficient  a  reflector 
as  the  connective-tissue  form. 

The  pigment  mantle  is  an  organ  whose  exact  function  is  rather 
obscure.  It  is  always  found  on  the  proximal  surface  of  the  organ,  and 
forms  there  a  thin  but  perfect  layer  of  branching  pigment  cells  developed 
^rom  a  connective-tissue  anlage.  In  the  cephalopod  mollusks  it  is 
formed  from  a  few  very  large  chromatophores  of  the  characteristic 
round  and  flattened  shape.  It  is  apparently  needless,  as  the  reflector 
protects  the  underlying  tissues  from  the  light,  and,  furthermore,  this 
pigment  cannot  reflect  light. 

The  lens  is  often  absent,  even  in  very  highly  developed  organs,  and 
more  particularly  in  those  that  are  found  in  land  forms.  In  its  most 
primitive  form  it  appears  as  a  slight  thickening  of  the  usually  transparent 
layers  that  are  placed  between  the  light  cells  and  the  exterior.  The 
cells  are  generally  of  a  rather  flattened  polygonal  shape  and  of  great 
transparency  and  refractive  power.  They  form  a  lens  that  brings  the 
light  to  a  focus  at  a  very  short  distance  from  its  point  of  origin,  and  not 
into  the  parallel  rays  that  would  at  first  be  expected. 

The  lens  is  often  composed  of  two  divisions,  a  proximal  and  a  distal 
one,  that  differ  from  one  another  slightly  in  their  texture.  The  mean- 
ing of  this  difference  is  not  known. 

The  structure  of  these  remarkable  organs  will  be  found  on  reflection 
to  be  remarkably  like  that  of  many  of  the  eyes  that  we  shall  study  in 
Chapter  XIII.  In  each  case  there  may  be  accessory  tissues  present  to 
handle  the  light  by  refraction,  reflection,  and  absorption.  The  two  im- 
portant differences  are  the  fact  that  in  the  one  case  the  specific  cells  give 
out  light  and  in  the  other  they  receive  it,  and  that  in  the  case  of  the  light 
organ  there  is  very  little  direct  connection  with  the  central  nervous  sys- 
tem, while  the  eye  is  from  its  very  nature  most  closely  connected  with  it. 

The  nature  of  the  light  has  been  somewhat  touched  upon,  but  it  may 
be  well  to  give  some  rather  more  exact  details  here.  It  has  been  the 
subject  of  some  very  exact  and  convincing  experiments  by  Langley  and 
Very  as  well  as  Young. 

Examination  with  the  spectroscope  has  shown  that  the  green  light, 
produced  by  the  common  firefly  of  the  United  States  and  by  several 
other  insects,  consists  of  those  rays  that  have  the  maximum  of  visibility 
and  the  minimum  of  heat  rays  and  ultra-violet  rays.  Its  appearance  is 
marked  by  the  absence  of  any  of  the  bands  that  show  a  deficiency  of 
waves  in  the  most  actinic  region.  This  perfection  shows  that  nearly  100 
per  cent  of  the  energy  is  transformed  into  light.  The  meaning  of  this 
becomes  clearer  when  we  consider  that  in  the  ordinary  gas  flame  only  a 
little  under  2  per  cent  of  the  energy  is  converted  into  light,  the  rest  being 
dissipated,  so  far  as  any  use  is  concerned,  as  low  heat  rays !  The  light 


TISSUES   OF  LIGHT-PRODUCTION  12$ 

of  other  colors,  that  is  to  be  seen  in  some  forms  of  animals,  has  not 
been  studied  as  yet,  and  we  are  unable  to  say  if  these  lights  are  as 
efficiently  produced  as  is  that  of  the  firefly.  As  has  already  been  said,  it 
can  easily  be  possible  that  some  animals  produce  a  light  that  we  cannot 
see  at  all,  its  waves  lying  outside  of  those  rays  that  we  can  see,  in  the 
non-actinic  part  of  the  spectrum. 

The  production  of  light  by  a  living  being  as  well  as  the  production  of 
electricity  seems  to  those  not  accustomed  to  the  idea  as  a  "  wonderful  " 
process.  Neither  one  is  any  more  wonderful  than  the  production  of 
motion  or  of  heat.  In  all  of  these  cases  we  can  easily  conceive  of  these 
functions  if  we  remember  that  protoplasm  itself  does  not  perform  them, 
but  is  restricted  to  the  r61e  of  making  or  secreting  certain  substances  that, 
when  they  are  brought  into  contact  with  oxygen  or  with  each  other, 
automatically  perform  the  act.  The  great  thing  to  understand  is  how 
the  protoplasm  is  able  to  continually  secrete  these  as  well  as  other  sub- 
stances under  other  than  some  "  vital  "  law. 

Many  animals  give  off  light  which  is  not  their  own,  but  which  is 
produced  by  some  bacteria  which  infest  their  bodies  or  which  are  present 
in  the  food  in  their  digestive  tracts.  This  can  be  seen  in  many  worms 
and  insects,  especially  some  midges  which  are  all  aglow  during  flight. 
This  condition  is  peculiarly  true  of  many  dead  Crustacea  where  a  sud- 
den exposure  to  oxygen  by  the  turning  over  of  driftwood  and  jetsam 
will  cause  them  to  all  light  up. 

Examples  of  Photogenetic  Tissues 

The  power  of  producing  light  is  found  in  one-celled  animals,  and 
the  protozoon,  Noctiluca  miliaris,  is  probably  the  best  known  example 
of  a  luminous,  one-celled  animal. 

This  tiny  creature  appears  on  the  surface  of  the  sea  in  countless 
numbers  in  some  localities.  Seen  in  the  daytime  under  the  microscope, 
it  shows  a  reticular  cytoplasm  through  which  a  great  number  of  granules 
are  scattered.  These  granules  are  probably  the  secretion  used  to  produce 
light.  We  shall  call  them  the  photochondria.  Around  the  nucleus  is 
an  area  of  cytoplasm  which  is  undifferentiated  in  that  it  does  not  pro- 
duce any  photochondria.  Its  surface  gives  off  very  many  fibrils,  which 
extend  radially  to  all  parts  of  the  periphery  where  they  are  attached. 
They  extend  through  the  photoplasm,  and  as  the  animal  always  contracts 
and  shines  at  the  same  time  it  will  not  be  far  amiss  to  conclude  that  the 
photochondria  act  also  as  myochondria  to  the  contractile  fibrils,  or  that 
both  kinds  of  granules  are  produced  by  the  cytoplasm  or  photoplasm. 

When  examined  at  night  with  a  low  magnifying  power,  the  animal 
gives  off  a  beautiful  light  that  appears  as  an  homogeneous  illumi- 


126  HISTOLOGY 

nation.  When  the  magnification  is  increased,  however,  it  can  be  seen 
that  the  light  is  given  off  from  myriads  of  tiny  dots  (Fig.  116).  These 
points  are  probably  the  same  as  the  granules  or  photochondria.  Any 
stimulus,  as  a  jar  or  an  electric  current,  causes  both  the  light  to  shine 
and  the  contractile  elements  to  shorten. 

The  Ctenophores  and  other  Coelenterata  also  show  light.  —  In  the 
common  Mnemeopsis,  careful  investigation  has  shown  that  the  light  is 
produced  in  the  "  ribs  "  on  the  sides  of  the  body.  It  was  not  possible  to 
show  which  cells  were  responsible  for  the  secretion  of  a  luciferase,  but 
the  substance  probably  appeared  in  some  contractile  elements.  As  in 
Noctiluca,  the  light  appeared  in  response  to  the  same  stimulus  that  causes 
contraction  and  cilliary  motion. 

A  case  of  light-production  in  the  Echinoderms  may  be  studied  in  an 
ophiurian.  The  genus  Ophiura,  of  these  creatures,  shows  two  species, 
telactes  and  phosphor  es,  which  are  luminous.  Again,  we  find  that  there 


FIG.  116.  — Phosphorescence  in  Noctiluca  milwris  Sur  (QUATREFAGES).     A  portion  of  the 
body  is  represented  with  numerous  scintillating  dots. 

are  no  exclusively  photogenetic  cells.  The  light  is  produced  in  some 
muscle  fibers  which  must  secrete,  therefore,  both  myochondria  and 
photochondria. 

The  Mollusks  have  Many  Light-producing  Tissues.  —  All  grades  of 
specialization  are  shown  and  we  shall  mention  three,  one  a  superficial 
organ,  and  the  others  organs  with  an  internal  secretion.  Pholas,  a 
plecypod  mollusk,  is  the  possessor  of  the  superficial  organ  which  ap- 
pears as  a  glandular  epithelium  that  discharges  a  luciferase  mixed  in 
mucus.  The  light  is  given  off  by  an  external  oxidization  of  this  mate- 
rial and  the  slime  continues  to  glow  for  some  time  after  its  discharge. 
The  epithelium  is  found  on  two  triangular  regions  on  the  inside  of  the 
mantle  and  on  two  cords  that  ascend  into  the  siphon,  as  well  as  in  some 
other  epithelia  that  do  not  produce  it  strongly. 

The  highly  specialized  light-organs  are  found  on  the  cephalopod  mol- 
lusks,  especially  those  which  live  in  deep  water.  They  show  a  great 
variety  of  forms,  and  we  shall  present  two.  One  of  them,  the  light-organ 


TISSUES   OF  LIGHT-PRODUCTION 


127 


of  Calliteuthes  reversa,  is  a  well-defined  and  easily  understood  structure 

found  on  the  outer  integument  of  this  dibranchiate  cephalopod.     Its 

secretion  is  used  in  situ  in  the  tissues, 

the  organ  being  inside  the  integument, 

instead  of  being  thrown  out  as  in  Pholas 

(Fig.  117).    Its  origin  is  not  known,  and 

it  may  be  either  ectodermal  or  mesoder- 

mal. 

Like  most  other  organs  of  this  nature, 
it  may  be  described  as  a  number  of  layers 
formed  in  a  limited  region  and  arranged 
on  a  distinct  proximodistal  axis.  The 
first  proximal  layer  consists  of  a  coat  of 
pigment,  thin  and  black,  which  covers 
the  inner  end  of  the  organ  like  a  cap. 
We  know  nothing  of  its  use  except  that 
it  probably  absorbs  superfluous  light. 

The  well  defined  and  thick  layer  of 
tissue  lying  distad  of  the  thin  pigment 
layer  is  composed  of  the  very  peculiar 
connective  tissue  which  has  been  differ- 
entiated to  reflect  light.  The  cells  are 
spindle-shaped,  and  lie  packed  with  their 
ends  interlocked  and  their  long  axis  at 
right  angles  to  the  direction  of  the  light 
ray  that  comes  from  the  nearest  light  cells.  The  substance  which  they 
have  in  their  bodies  to  reflect  light  is  possibly  some  urate  in  a  very 
finely  divided  state. 

Still  distad  of  this  tapetum  or  reflector  is  the  photogenic  layer,  com- 
posed of  a  single  row  of  columnar  cells  lying  in  a  row  that  conforms  to 
the  shape  of  the  last  two  layers.  It  is  as  thick  as  the  many-layered  re- 
flector, but  smaller  by  necessity  of  its  distal  and  inner  position.  Its 
shape,  as  that  also  of  the  last  two  layers,  may  be  roughly  likened  to  a 
horseshoe.  The  cells  which  compose  it  are  long,  thin  gland  cells,  each 
with  a  small  nucleus  in  the  proximal  part,  and  the  secretion  region  and 
storage  region  in  the  distal  cytoplasm.  The  blood  supply  is  evidently 
connected  in  some  way  with  the  reflecting  tissue. 

Distal  from  the  light-gland  is  found  the  lens-tissue.  This  is  rather 
remarkable  for  its  apparent  lack  of  homogeneity.  It  is  made  up  of  a 
number  of  heavy  fibers  that  anastomose  into  a  reticular  mass.  This 
mass  forms  two  rounded  and  connected  areas,  the  smaller  of  which  lies  in 
the  concavity  on  the  distal  side  of  the  light  tissue.  This  small  portion 
is  directly  continuous  with  the  far  larger  outer  or  distal  part,  which  is 


re/— 


P9-— 


FIG.  117.  —  Section  through  main  axis 
of  light  organ  of  Calliteuthes  reversa. 
lu.c.,  luminous  cells;  /.,  lens;  ref., 
reflector;  pg.,  pigment;  in.,  integu- 
ment. (After  C.  CHUN.) 


128  HISTOLOGY 

made  up  of  larger  cells,  and  fits,  as  the.  real  lens,  on  all  the  other  layers 
combined.  The  reticulum  is  composed  of  meshes  that  are  drawn  out 
in  the  proximo-distal  axis  of  the  entire  organ  and  in  the  direction  of 
light  transmission.  If  the  spaces  in  the  reticulum  are  filled  in  life 
with  a  fluid,  especially  if  that  fluid  is  of  a  high  index  of  refraction,  as 
it  probably  is,  the  lens  would  act,  as  a  whole,  very  efficiently.  The 
central  axis  of  this  organ  is  directed  at  a  rather  sharp  angle  to  the  body 

surface.  It  is  possible 
that,  in  life,  this  angle 
can  be  changed  at  the 
animal's  convenience. 

One  more  cephalopod 
light-organ  must  be  de- 
. .-       scribed    as    an   example 
1  jyW      of  the  highest  complexity 
and  specialization.    This 
is    the    luminous    organ 
found  in  the  integument 
of  Abraliopsis  (Fig.  118). 
This    organ    occupies   a 
FIG.  1 1 8.— Section  through  principal  axis  of  light-organ  of  nearly  spherical  sac  near 

Abraliopsis.     lu.c.,  luminous  cells;  I.,  lens;  ref.,  reflector;   the  Surface,  and  has  rep- 
pg.c.,  pigment  cells.     (After  C.  CHUN.)  ,    .      .  . 

resented  in  its  composi- 
tion all  the  structures  that  are  found  in  almost  any  light-organ.  Its 
symmetrical  central  axis,  passing  from  proximal  to  distal  end,  lies 
at  right  angles  to  the  body  surface.  A  crescent-shaped  blood  space 
occupies  the  posterior  part  of  the  sac.  This  space  is  divided  into 
compartments  by  a  number  of  thin  connective-tissue  membranes. 
The  possibilities  of  blood  circulation  through  these  lacunae  are  not 
known. 

Lying  between  this  blood  space  and  the  other  central  organs  is  a 
layer  of  pigment  cells.  They  are  symmetrically  placed,  one  in  the 
central  axis  and  the  other  two  slightly  overlapping  it  at  the  side. 
Together  they  represent  a  cup-shaped  figure,  just  covering  and  embrac- 
ing, proximally,  a  mass  of  plasma  which  contains  between  its  two  parts 
the  strangely  formed  reflectors  which,  like  the  pigment  elements,  are 
three  in  number. 

These  reflectors  are  a  puzzle  in  that  their  positions  do  not  seem  to 
be  mechanically  adapted  for  the  best  or  even  for  good  results.  They 
are  each  composed  of  a  plate  of  parallel,  flat  cells.  In  the  middle  one, 
these  cells  are  curved  into  a  semicircle,  which  would  be  a  shape  of 
reflector  not  well  calculated  to  direct  the  rays  all  outward.  The  two 
lateral  bundles  in  our  figure  represent  a  circular  reflector,  which  also  is 


TISSUES   OF  LIGHT-PRODUCTION  129 

seen  in  our  section  to  be  of  a  poor  shape  to  reflect  the  lateral  rays  and 
those  lost  in  the  central  part  of  the  apparatus. 

In  the  space  between  the  pigment  and  reflectors  is  a  connective  tissue 
of  unknown  function.  Also  in  the  small  space  that  lies  within  the  arms 
of  the  reflectors  we  see  another  and  somewhat  fibrous  connective  tissue 
instead  of  the  photogenetic  cells  as  in  most  organs  similar  to  this  one. 

The  light-producing  cells  form  a  disk-like  mass,  somewhat  thickened 
and  very  slightly  concave  on  the  distal  surface.  The  cells  themselves 
are  not  arranged  in  columnar  order  as  in  the  last  specimen,  but  are  packed 
together  in  several  thicknesses  with  vacuole-like  spaces  in  the  mass. 
The  nuclei  are  branched  and  large.  Cell  boundaries  are  entirely  wanting. 

In  the  front  of  the  light-cell  mass  comes  the  lens.  It  is  of  the  same 
shape  (a  disk)  as  the  light-cell  mass,  but  nearly  four  times  as  thick  in 
the  proximo-distad  direction.  Its  proximal  surface  is  slightly  convex 
to  fit  the  concave  surface  of  the  light  mass,  and  its  anterior  surface  is 
the  same  shape  to  agree  with  the  contour  of  the  entire  sac,  against  whose 
wall  it  lies.  Its  substance  is  made  up  of  fibrous  or  plate-like  cells  packed 
in  bundles,  all  of  which  lie  at  right  angles  to  the  axis  and  to  the  light  rays. 
The  nuclei  are  small  and  scattered. 

A  circular  ring  of  reticular  connective  tissue  surrounds  the  lens  on 
all  sides.  It  is  about  as  thick  as  the  lens  mass,  and  taken  together  with 
it,  forms  a  large  disk,  which  occupies  the  front  or  distal  part  of  the  eye. 
The  lens  does  not  touch  the  actual  body  surface,  but  between  it  and  the 
thin,  tough  cornea  is  a  space,  probably  to  contain  a  fluid. 

Several  Worms  possess  a  Fairly  Strong  and  Steady  Luminosity. — One 
marine  Annelid  gives  a  bright  spark  during  the  mating  season.  An 
earthworm  shows  a  general  light  given  off  in  the  slime,  as  is  done  in  Pholas. 

Light-organs  are  rare  among  the  Crustacea,  but  are  of  interest  because 
of  the  relationship  of  their  bearers  to  the  insects.  Some  Copepoda  show 
a  luminosity  which  is  produced  by  granules  of  a  secreted  material  that 
is  thrown  off.  Woltereck  has  mentioned  a  possible  case  among  the 
Crustacea  in  the  deep-sea  Amphipod,Scypholanceola,  which  has  two  pairs 
of  peculiar  organs  on  the  head.  No  adequate  description  of  the  histology 
of  these  organs  was  accessible  to  the  writers. 

The  best  case  of  luminosity  in  a  Crustacean  is  that  of  the  Schyzopod 
group,  Euphusida,  a  pelagic  family  with  many  representatives.  The 
organ,  as  found  in  the  form  Nyctiphanes  Norvegica,  has  been  described 
by  Vallentin  and  Cunningham  as  well  as  discussed  by  Giesbrecht,and  we 
shall  use  it  as  a  type.  The  light-organs,  or  photos pheria,  of  this  animal 
show  two  forms.  That  found  on  the  eye  pedicle  is  not  as  well  developed 
as  those  found  along  the  sides  of  the  body.  The  one  found  on  the  side 
of  the  first  abdominal  segment  is  characteristic  and  fully  developed.  It 
is  complex  and  highly  efficient  (Fig.  119). 


130 


HISTOLOGY 


The  chief  portion  of  this  organ  consists  of  a  nearly  spherical  mass 
of  tissue  which  is  not  moved  by  muscles,  as  has  been  asserted  by  some 

authorities,  but  lies  loosely 
embedded  in  the  tissues 
of  the  body  wall  and 
bounded  distally  by  the 
cuticle.  The  proximal 
bounding  layer  is  con- 
nective tissue,  from  its 
appearance,  and  is  devel- 
oped to  act  as  the  re- 
flector by  the  plate-like 
arrangement  of  its  lamellae, 
which  are  thin,  and  placed 
in  a  parallel  cup-shaped 
layer  that  embraces  the 
rest  of  the  organ  on  its 
inner  end.  There  are 
but  few  nuclei  scattered 
through  this  layer.  This 
reflector  is  thin,  and  on 
its  outer  or  proximal 

FIG.  119.  —Light-organ  of  Nyctiphanes  Norvegica.  lu.c..  Surface  it  is  Covered  with 
luminous  cells;  c.,  cornea;  /.,  lens;  ref.,  reflector;  ft.,  fi-  ^  layer  of  red  pigment 
brillar  mass.  (After  VALLENTIN  and  CUNNINGHAM.)  ,, 

The  next  layer  distad  is  a  columnar  layer  of  thick,  heavy  cells  that 
form  a  cup-like  structure  lying  in  and  touching  intimately,  the  reflector. 
The  nuclei  of  these  cells  lie  mostly  distad  in  the  layer,  although  some  few 
are  found  proximally.  This  is  apparently  because  the  layer  is  not  en- 
tirely single,  but  partially  stratified  in  some  places.  These  large  cells  show 
a  cytoplasm  that  is  full  of  some  granular  secretion,  and  they  are  probably 
the  gland  cells  which  secrete  the  light  substance  or  luciferase.  Con- 
siderable difference  of  opinion  exists  as  to  just  which  cells  were  the 
actual  source  of  the  light,  and  it  is  possible  that  a  light  substance  may 
be  produced  in  this  layer  and  then  discharged  against  the  reflector  or, 
more  probably,  into  the  inner  fibrillar  mass,  to  be  there  oxidized  and 
made  to  shine. 

This  peculiar  mass  lies  in  the  cup-like  embrace  of  the  last  layer  de- 
scribed, the  gland  cells.  It  is  made  up  of  several  bundles  crossing  each 
other  at  an  angle,  and  its  outer  (proximal)  layer  forms  a  series  of  shorter 
radial  rods.  Nuclei  are  very  scarce  in  connection  with  it,  and  where 
found  on  its  edge  evidently  belong  to  other  tissues.  Placed  above  this 
mass  distally  and  overlapping  the  glandular  layer  is  a  single,  very  per- 


TISSUES   OF  LIGHT-PRODUCTION 


feet  lens.     Its  shape  can  be  better  understood  by  a  glance  at  the  figure 
than  a  page  of  description. 

Stretched  over  the  lens  and  overlapping  even  the  reflector  is  a  thick 
corneal  layer  which  is  double  on  its  edges  and  single  in  the  middle.  The 
lower  part  of  this  double  region  is  applied  to  the  edge  of  the  reflector. 
It  is  heavily  striated  epithelium,  and  continues  as  a  sheet  of  loose  cells, 
which  form  one  of  the  few  means  by  which  the  whole  organ  is  connected 
with  the  tissues  among  which  it  lies.  Large  blood  spaces  effect  the  iso- 
lation. Above  this  cornea  comes  the  hypodermis  and  its  layer  of  cuticle. 
These  structures  are  continuous  with  those  on  the  rest  of  the  body. 

The  origin  of  these  tissues  has  not  been  worked  out.  Being  an  Ar- 
thropod, and  remembering  that  in  the  insects  the  light  tissue  is  developed 
from  the  mesoderm,  it  is  plausible  to  consider  these  structures  as  also 
mesodermal.  But  the  general  appearance  of  the  various  layers, 
especially  in  a  younger  organ,  gives  a  different  aspect  to  the  matter 
and  leads  one  to  believe  that  the  whole  organ,  with  the  exception 
of  the  reflector,  is  formed  by  an  involved  invagination  or  delamination 
of  the  hypodermis  in  the  embryo. 

The  second  group  of  Arthrop- 
oda,  the  insects,  exhibit  sev- 
eral very  efficient  tissues  that 
produce  light.  These  organs  are 
supposed  to  be  mesodermal  in 
origin,  and  probably  are  modified 
fat  bodies.  A  reflector  is  usually 
present,  and  consists  of  a  layer  of 
closely  packed  cells  that  have  the 
same  origin  as  the  gland  cells. 
The  light  secretion  is  alway  sused 
in  situ,  in  the  cytoplasm  of  the 
cells  which  produced  it.  It  is 
surprising  that  a  lens  is  always 
missing  unless  some  of  the  cutic- 
ular  structures  may  be  so  inter- 
preted. While  several  sorts  of  light- 
organs  occur,  more  or  less  simple  in 
structure,  the  more  complex  ones 
may  always  be  easily  compared 
with  the  simpler.  We  shall  begin 
with  the  study  of  a  simple  form. 

The  light-organ  of  a  Lampyrid  will  furnish  such  a  type,  and  that  of 
Lampyris  splendidula  and  noctiluca,  as  described  by  Bongardt,  will  serve 
our  purpose. 


FIG.  120.  —  Part  of  a  section  through  the  light- 
producing  tissue  of  Lampyris  splendidula, 
showing  a  single  tracheal  end-cell  (tr.e.c.) 
into  which  a  terminal  twig  (ter.t.)  of  a  tra 
cheal  tube  enters.  This  twig  gives  off  five 
tracheoles  (trl.).  The  general  histology  of  this 
tissue  is  much  like  that  in  the  next  figure. 
(After  BONGARDT.) 


132 


HISTOLOGY 


The  organ  consists  of  a  round  mass  of  simple  polyhedral  cells,  and  is 
situated  in  the  abdomen  facing  its  ventral  surface.  Those  of  the  cells 
which  are  nearest  that  surface  are  the  light-cells,  and  secrete  the  light  sub- 
stance or  luciferase  in  their  abundant  cytoplasm.  The  inner  and  dorsal 
layer  secretes,  instead,  a  white  material  (ammonium  urate)  and  serves 

to  reflect  all  light  rays  downward. 
The  light  tissue  is  most  abun- 
dantly supplied  with  trachea  to 
bring  in  air.  The  tracheae  branch 
freely,  and  the  terminal  twigs  are 
distributed  so  that  each  group 
of  several  cells  has  one  of  these 
twigs  in  its  midst  (Fig.  120). 

The  relations  of  such  a  ter- 
minal twig  to  the  group  it  sup- 
plies are  peculiar.  It  enters  into 
a  central  cell  of  the  group  which 
is  called  the  tracked  end-cell,  and 
in  this  cell  it  gives  off  from  its 
end  a  number  of  fine  air  capilla- 
ries, the  tracheoles,  that  branch 
out  and  pass  through  the  cell 
group  between  the  cells.  From 
each  end  twig  there  are  three  to 
five  or  more  of  these  tracheoles, 
finely  ringed,  and  each  one  soon 
diminishes  in  size  to  a  still 
smaller  and  smooth,  capillary 
tube,  which  anastomoses  with 
some  of  the  tracheoles  coming 
from  other  end-cells.  In  the 
first  part  of  their  course,  at  least, 
the  tracheoles  are  followed  and 
surrounded  by  protoplasmic 
processes  of  the  tracheal  end- 
cells.  In  the  latter  part  of  their 
way  they  apparently  pass  be- 
tween the  light-cells  and  never 
enter  their  cytoplasm.  Bongardt 
was  not  able  to  find  any  tracheal 
nuclei  in  this  part  of  their  course. 
The  tracheal  end-cells,  until  proved  otherwise,  must  be  looked  upon 
as  specialized  hypodermal  cells  that  have  grown  into  the  tissue  with  the 


FlG.  121. — Vertical  section  through  the  median 
abdominal  light-organ  of  Pyrophorus:  ref.c., 
reflector  cells;  lum.c.,  luminous  cells;  mus.f., 
muscle  fibers  in  transection.  Tracheal  details 
not  shown;  they  may  be  represented  in  general 
by  those  in  the  preceding  figure.  X  850. 


TISSUES   OF  LIGHT-PRODUCTION  133 

tracheae  to  form  and  maintain  these  necessary  tubes.  Theoretically  a 
thin  layer  of  their  cytoplasm  should  follow  all  branches  of  any  tracheal 
ending.  As  so  far  actually  seen,  it  follows  these  branches  only  part 
way,  and  the  finest  branches  must  be  formed,  if  this  is  true,  by  the 
activities  of  the  light-cells  which  lie  in  a  trophic  contact  with  them. 

It  should  be  noticed  that,  in  this  tissue,  the  end-cells  and  light-cells 
are  irregularly  grouped  in  alveoli  of  sufficient  size  throughout  the  light 
tissue.  This  is  also  true  of  the  elatrid  beetle,  Pyrophorus,  of  Jamaica, 
from  whose  median,  abdominal  organ,  the  Figure  121  was  drawn. 

We  shall  now  study  briefly  the  luminous  tissue  of  our  common 
American  firefly,  Photinus  marginalis,  which  has  been  recently  worked 
out  by  Miss  Townsend. 

In  this  insect  are  found  the  same  two  layers,  a  light-producing  layer 
next  the  integument,  and  proximal  to  this  a  reflecting  layer  to  send  out 
all  rays.  Here,  too,  the  trachea  come  down  through  the  reflecting  layer 
and  enter  the  photogenetic  layer,  but  not  so  haphazardly  as  in  Lampyris. 
They  descend  at  regularly  spaced  intervals  into  little  cylinders  which 
reach  vertically  the  whole  distance  from  reflector  to  integument  through 
the  light-cell  layer  (Fig.  122).  In  their  downward  course  they  give  off 
laterally  about  a  hundred  terminal  twigs,  which  each  pass  directly  into 
a  cell  lying  next  to  the  main  tracheae.  These  cells  form  the  walls  of  the 
cylinder,  and  from  the  way  that  the  lateral  terminal  twigs  break  up  inside 
of  them,  we  can  recognize  them  at  once  as  the  tracheal  end-cells  that  were 
found  in  Lampyris.  The  only  difference  is  that  in  Lampyris  the  end- 
cells  were  scattered  among  the  light-cells  at  random,  so  long  as  each  one 
was  the  center  of  a  round  group  of  the  light-cells  that  it  could  supply  with 
oxygen,  while  in  Photinus  there  is  an  organization  of  the  end-cells  into 
a  layer  that  surrounds  each  main  tracheal  stem  as  a  cylindrical  tube, 
and  outside  of  and  between  these  cylinders  lies  the  mass  of  light- 
cells. 

There  is  here  an  evident  structural  economy.  The  length  of  tracheal 
tube  is  shorter,  and  the  oxygen-laden  air  can  be  brought  in  larger  quan- 
tities and  more  suddenly  and  efficiently  into  contact  with  the  light-cells. 
The  result  is  also  evident  when  the  insects  are  observed  in  life'.  The 
Photinus  gives  a  quick,  short,  and  dazzling  flash,  while  a  glowworm  (the 
common  ground  glowworm,  a  lampyrid  larva),  with  its  diffuse  form  of 
tissue,  glows  slowly  and  softly  for  a  few  seconds. 

Several  centipedes  show  a  light  which  is  produced  by  a  discharged 
external  secretion.  It  is  thrown  and  rubbed  on  their  enemies  as  a  slime 
containing  tiny  granules. 

Among  the  tunicates  are  some  members  of  the  group  that  are  pro- 
vided with  photogenetic  organs.  Pyrosoma  is  one  in  which  this  is  very 
evident.  The  tissues  consist  of  two  cell  masses  in  the  integument,  one 


134 


HISTOLOGY 


on  each  side  of  the  body,  near  the  siphon.    The  cells  resemble  fat  cells 
in  form,  the  content  of  the  vacuole  being  luciferase. 

The  fishes  are  among  the  animals  that  produce  light.  The  power  is 
found  in  some  simple  as  well  as  some  highly  developed  forms  in  this  class. 
As  is  true  of  the  other  classes,  but  few  of  the  species  and  genera  are  pho- 


tr.e.c. 


lum.  c. 

FIG.  122.  —  Part  of  a  vertical  section  through  the  luminous  organ  of  Photinus  marginalis. 
lum.c.,  luminous  cells;  ref.c.,  reflector  cells;  tr.,  tracheae;  ter.t.,  terminal  twig  of  tra- 
cheae, tracheoles  not  visible;  tr.e.c.,  trachea!  end-cells;  tr.ep.,  tracheal  epithelium. 
X  1000. 

togenetic,  in  proportion  to  the  great  numbers  that  exist.  The  organs  are 
found  in  perhaps  ten  selachian  forms  and  in  very  many  more  teleosts  of 
most  of  the  larger  groups.  The  number  of  light- producing  teleost  species 
probably  reaches  into  the  hundreds. 

The  selachians  show  the  simplest  forms,  and  we  shall  describe,  as  an 
example,  the  light  tissues  of  Spinax  niger,  a  small  Japanese  shark  which 
glows  brightly  in  the  dark.  The  skin  of  this  fish  shows,  morphologically, 


TISSUES   OF  LIGHT-PRODUCTION 


135 


a  number  of  regions  where  the  light  may  be  seen.  A  vertical  section  of 
one  of  these  areas  from  the  belly  shows  the  epithelial  structure  common 
to  most  selachian  fishes,  a  moderately  thin  stratified  layer  with  large 
mucous  cells  showing  in  its  outer  part  and  a  few  widely  spaced  spines. 
The  large  hollow  mucous  cells  usually  contain  round  homogeneous  con- 
cretions. The  luminous  organs  are  found  lying  between  the  spines 
and  are  much  more  numerous  than  the  spines  are.  They  can  be  picked 
out  instantly  as  thickened  regions  of  the  epithelium  into  which  branching 
pigment  cells  have  wandered.  In  this  region  the  basal  layer  of  the  epithe- 
lium is  invagi- 
nated  into  the 
cutis  as  a  pocket 
of  simple  shape 
with  an  opening 
that  is  not  con- 
stricted. 

The  six  or 
eight  cells  which 
occupy  the  bot- 
tom  of  this 
pocket  are  en- 
larged and  their 
distal  ends  are 
collected  into  a 
central  mass. 
Since  the  distal 
end  of  each  one 
is  filled  by  sev- 
eral photochon- 
dria  or  masses 
of  light  material 
secreted  by  the 
cells,  this  mass 

is  the  point  from  FIG.  123.  —  A  light-organ  from  the  skin  of  Spinax  niger.     b.m.,  base- 

wViirVi     tVip     lio-Vit  rnent  membrane  of   invaginated  basal  epithelium;   lum.c.,  luminous 

U&UL  cells;  I.e.,  lens  cells  with  intracellular  lens  secretions;  pg.c.,  pigment 

emanates,        and  cells;  lum.sec.,  luminous  secretion;  art.,  artifact.     X  580. 

.the  cells  are  the 

specific  photogenetic  cells  of  the  tissue.  Such  cells  of  the  same  basal 
layer  as  are  found  on  the  sides  of  the  opening  are  not  specialized, 
and  those  of  their  proliferated  descendants  which  stretch  across  the 
opening  of  the  imagination  are  only  somewhat  flattened  and  made 
transparent  to  permit  of  the  light's  free  exit.  In  the  specimen  from 
which  the  drawing  was  made  the  poor  alcohol  fixation  had  caused 


art. 


Pff.c. 


lum.  c. 


136 


HISTOLOGY 


some  shrinking,  and  the   artificial  separation  of  some  of  the  cells  is 
reproduced  in  Figure  123  at  art. 

The  two  or  three  large  cells,  which  lie  above  this  central  layer  and 
usually  also  directly  above  one  another,  are  specialized  to  form  in  their 
cytoplasm  very  large,  solid  concretions  which  may  act  as  a  lens  to  con- 
centrate the  light  as  it  passes  out.  The  outer  of  these  cells  are  the  larger, 
and  have  the  concretion  developed  in  size  almost  to  the  point  that  a  fat 
globule  is  sometimes  developed  in  its  cell.  In  the  inner  cell  the  concre- 


lum.  ep. 


FIG.  124.  —  Section  of  light-organ  of  a  deep-sea  fish,  Gigantactus.  lum.ep.,  luminous  epithe- 
lium; ref.,  reflector;  pg.,  pigment  layer;  dt.,  duct  with  enlarged  chamber.  (After  A. 
BRAUER.) 

tion  is  evidently  in  an  earlier  stage  of  development,  and  that  brings  up  the 
question  as  to  whether  the  light  cells  or  the  lens  cells  are  renewed  by 
growth  processes  or  not.  Their  origin  from  a  stratified  epithelium  would 
lead  one  to  think  that  they  were  constantly  worn  out  and  replaced, 
while  the  position  of  the  lens  cells  would  suggest  that  the  latter  must 
remain  where  they  are  unless  they  go  through  a  stage  in  which  they 
are  light  cells  and  are  finally  thrown  off.  There  is  probably  no  renewal. 
A  regeneration,  in  case  of  such  abrasion  as  must  often  occur,  is  pos- 
sible. 


TISSUES   OF  LIGHT-PRODUCTION 


137 


lU.  C, 


A  much  greater  variety  of  light-organs  is  to  be  found  in  the  teleost 
fishes.  Space  forbids  a  description  of  all  the  types,  several  of  the  most 
important  of  which  will  be  given. 

The  single  light-organ  on  the  angling  fin  ray  of  Gigantactus,  a  deep- 
sea  fish,  is  a  simple  type.  It  consists  of  an  imagination  of  the  stratified 
epithelium  into  the  connective  tissues  of  the  bulb  on  the  end  of  the  ray. 
This  invagination  consists  of  a  deep  round  sac  at  the  fundus,  an  inter- 
mediate vestibule,  which  is  wide  but  short,  and  a  tube  leading  from  the 
vestibule  to  the  exterior  (Fig.  124). 

The  deep  sac  is  the  important  structure.  The  epithelium  which  lines 
it  is  still  stratified,  but  during  its  proliferation  each  of  its  cells  secretes  the 
light  substance,  and  when  finally  cast  off  at  the  surface  it  degenerates 
and  the  light  substance  is  thrown  into  the  lumen. 

From  the  lumen  it  must  be  slowly  forced  out  into  the  vestibule  and 
from  the  vestibule  it  must  pass  into  the  surrounding  wrater  through  the 
tube.  It  is  probably  oxidized  in  the  vestibule  and  made  to  give  up  its 
light,  so  that  only  the  combustion  products  pass  into  the  water.  The 
organ  thus  acts  as  an  external  or  superficial  tissue  of  photogenesis,  a 
comparatively  rare  form,  especially  among  the  higher  animals.  The 
greater  part  of  these  teleost  light- 
organs,  however,  are  internal, 
meaning  that  the  secretion  is  not 
discharged,  but  is  used  in  situ  hi 
or  near  the  cells  that  produced  it, 
as  was  the  case  in  Spinax. 

Passing  over  the  simplest 
forms  that  show  but  a  few 
specific  cells  inclosed  proximally 
by  a  pigment  layer,  we  shall 
examine  the  type  of  luminous 
organ  found  on  Chauliodus. 

This  consists  of  a  proximal,  CUp-  FIG.  125.— A  light-organ  of  Chauliodus.  lu.c., 
Shaped  pigment  mantle  (Fig.  125)  luminous  cells;  pg.,  pigment  layer;  /.,  lens. 
,.  *,  ,.  ,,  ,  .  i  ,  r  (After  A.  BRAUER.) 

lined  distally  by  a  single  layer  of 

columnar  cells  whose  small  basal  ends  contain  a  nucleus  and  whose  long 

distal  ends  are  filled  with  the  secretion. 

Held  in  the  hollow  of  this  cup-shaped  gland  layer  is  the  solid  mass 
of  cells  which,  in  life,  are  transparent  and  refractive  and  act  as  the  lens. 
This  mass  has  an  outer  layer  somewhat  separated  from  the  rest  by  its 
columnar  arrangement  but  functionally  a  part  of  it. 

Outside  the  lens  is  the  connective-tissue  layer  of  the  skin.  Its  rela- 
tion to  the  remainder  of  the  organ  is,  functionally,  that  of  a  cornea. 
Morphologically  the  lens  and  gland  cells  came  from  the  epithelium  and 


138 


HISTOLOGY 


P9- 


b.  m. 


lum.    or. 

FIG.  126.  —  A  first  stage  in  the 
development  of  a  light- organ  of 
Porichthys.  b.m.,  basement 
membrane;  mu.c.,  mucous 
cells;  pg.c.,  pigment  cell  in  con- 
nective tissue;  lum.or.,  anlage 
of  luminous  organ.  (After  C. 
W.  GREENE.) 


were  separated  from  their  parent  tissue  during  the  embryonic  history  by 
the  connective  tissue.  This  process  has  been  carefully  described  by 
Greene  in  Porichthys.  We  shall  study  the 
luminous  organ  of  Porichthys.  by  tracing  its 
histogenesis,  which  is  accessible  and  under- 
stood. 

The  skin  of  an  embryonic  Porichthys  shows 
a  thin  stratified  epithelium  much  like  that  of 
the  shark.  Of  course  no  spines  are  present  and 
the  mucous  cells  are  somewhat  numerous. 

At  points  in  the  basal  layer  of  this  epithe- 
lium a  crowding  of  the  nuclei  will  be  noticed, 
and  this  is  soon  followed  by  an  invagination 
of  the  whole  layer  as  well  as  a  general  thick- 
ening of  the  epithelium  at  this  point.  Fig- 
ures 126  and  127  show  two  invagination 
stages  in  such  a  region  after  the  invagination 
has  proceeded  to  some  extent,  and  the  structure  resembles  the  light- 
organ  of  Spinax  except  that  its  cells  are  not  differentiated  from  one 
another,  and  the  outlying  pigment  cells,  which  are  much  increased  in 
size  and  number,  have  not  moved  into  the  epithelium. 

The  process  now  proceeds  farther  than  it  did  in  Spinax  by  the  con- 
striction of  the  mouth  of  the  in- 
vagination, as  in  Figure  128, 
where  the  rounded  mass  of  cells 
is  separated  from  its  parent  epi- 
thelium by  the  ingrowing  con- 
nective tissue.  Here  can  also  be 
seen  the  beginning  of  a  differen- 
tiation of  the  proximal  from  the 
distal  cells  of  the  mass.  The 
first  are  becoming  granular  and 
vacuoles  are  appearing  in  their 
cytoplasm.  Pigment  cells  are 
also  present  and  usually  more 
numerous  than  in  the  specimen 

lum.  or.  drawn.    They  do  not  touch  the 

FIG.  127.  — Later  stage  than  Figure  126  of  devel-   invaginated  mass  but    remain 
sTm?  H(Aftergcnwf  oSS?T    Le"ering  *"   constantly  separated  from  it  by 

a  part  of  the  connective  tissue. 

This  connective  tissue  also  shows  the  beginning  of  a  differentiation. 
It  forms  plates  and  fibers  which  lie  parallel  with  the  proximal  outline  of 
the  invaginated  cell  mass  and  form  a  distinct  layer. 


TISSUES   OF  LIGHT-PRODUCTION 


139 


The  final  development  is  seen  in  Figure  129.  The  invaginated  cell 
mass  is  differentiated  into  a  proxi- 
mal layer  which  produces  luciferase, 
and  a  distal  portion  which  has  grown 
transparent  and  refractive  to  be  used 
as  the  lens.  The  proximal  connec- 
tive-tissue layer  has  acquired  a  dense 
white  appearance  that  enables  it  to 
be  used  as  a  reflector,  and  behind  it 
lie  the  pigment  cells  in  their  usual 
position  in  a  light-organ. 

The  organ  pictured  in  section,  as 
Figure  129,  is  not  yet  fully  formed. 
Greene  presents  a  figure  of  an  adult 
photosphere,  but  this  stage  in  Figure 

129   is   sufficient    for    the    purpose    of    FIG.  128.  — Still  more  advanced  stage  than 

understanding  the  fully  formed  organ,      Fi.gure  "i<&  Parickthys  light-organ.   LU- 

°  J  '        minous  cells  (lu.c.)  of  organ  beginning  to 

Which  differs  from  it  Only  in  size  and        differentiate  from  lens  cells  (I.e.),  also  traces 

some  trifling  points  of  form.  of  rT7eflector  W-\  °the£  le"er'ne the  same 

as  Figure  126.    (After  C.  W.  GREENE.) 

Light-organs  have  been  described 

in  the  young  of  some  birds,  especially  on  the  edge  of  the  mouth  in  the 

nestlings  of  an  Aus- 
tralian finch.  These 
were  rightly  supposed 
to  be  of  use  in  guiding 
the  parent  when  deliv- 
ering food  to  its  young 
when  the  time  or  place 
rendered  the  nest 
dark. 

Chun  found  that  a 
weak  light  appeared 
to  be  given  off  from 
the  papillae  itf  an  or- 
dinary darkness  until 
the  room  was  made 
absolutely  dark,  when 
no  glow  whatever  was 
apparent.  Also  his- 
tological  investigation 
showed  the  entire  ab- 
sence of  any  cells  which  appeared  to  secrete  a  luciferase.  It  was 
therefore  concluded  that  the  organ  was  a  structure  that  could  collect 


FIG.  129.  —  Young  but  fully  formed  light-organ  of  Porichthys. 
lu.c.,  luminous  cells;  ref.,  reflector;  I.e.,  lens  cells.  Other  let- 
tering the  same  as  Figure  126.  (After  C.  W.  GREENE.) 


140  HISTOLOGY 

and  make  visible  weak  rays  of  light  by  reflection  and,  perhaps,  by  con- 
densation, but  which  could  not  produce  light.  This  same  condition 
holds  true  of  the  eyes  of  many  animals. 

Technic.  — The  tissues  are  fairly  easy  to  section  and  stain,  with  the 
exception  of  those  that  are  covered  with  a  hard  chitin,  which  must  be 
removed.  It  is  best  to  embed,  and  after  scraping  down  to  the  chitin,  to 
remove  it  while  the  specimen  is  in  the  block  and  then  re -embed.  The 
study  of  some  of  the  details  has  been  helped  by  special  methods.  The 
ultimate  branches  of  the  air  passages  have  been  brought  out  by  the  use 
of  osmic  acid  on  the  living  insect.  This  has  entered  the  tracheae  and 
blackened  the  tracheoles  to  the  exclusion  of  all  but  the  most  immediately 
surrounding  tissues.  Also  many  of  these  tissues  have  been  worked  out 
from  crude  alcoholic  material,  owing  to  the  rarity  of  some  of  the  speci- 
mens. Flemming's  fluid  is  probably  the  best  general  fixative  for  this 
class  of  tissue. 

LITERATURE 

BURKHARDT,  R.      "  Luminous  Organs  of  Selachian  Fishes,"  Ann.  and  Mag.  Nat.  Hist., 

7th  series,  Vol.  VI,  1900. 
JOHANN,  LEOPOLD.     "  tiber  eigentumliche  epithelial  Gebilde  (Lichtorgane  bei  Spinax 

niger},"  Zeits.  f.  Wiss.  Zool.,  Band  LXVI,  1899. 
GREENE,  C.  W.      "Light-organs  of  the  fish,  Porichthys.    Histogenesis,"  Journ.  Morph., 

Vol.  XV. 

WATASE,  S.      "  Animal  Luminosity,"  Biol.  Led.  Woods  Holl,  1898. 
BRAUER,  A.     "  Uber  die  Leuchtorgane  der  Knochenfische,"  Verh.  deutsch.  Zool.  GeselL, 

Band  XIV,  S.  16,  1904. 
CHUN,  C.     "  IJber  Leurhtorgane  und  Augen  von  Tiefsee-Cephalopoden,"  Verh.  deutsch. 

Zool.  GeselL,  Band  XIII,  S.  67-91. 


CHAPTER   XI 
TISSUES  WHICH   PRODUCE  HEAT 

IN  the  last  three  chapters  we  have  been  studying  tissues  that  were 
differentiated  and  organized  to  produce  three  forms  of  energy  for  the  use 
of  the  organism.  These  were  motion,  electricity,  and  light.  Protoplasm 
also  produces  heat,  and  does  it  in  the  same  way  that  it  generates  light  and 
electricity, — by  the  secretion  of  substances  that,  when  combined  with 
oxygen  or  some  other  reducing  agent,  generate  the  heat  required.  Where 
known  at  all,  the  heat  secretion  appears  in  the  form  of  granules  which, 
could  they  be  specifically  identified,  might  be  called  thermochondria. 

As  the  heat  is  produced  by  the  oxidation  of  particles,  it  is  probable 
that  when  first  generated  it  is  concentrated  into  very  small  areas  of  the 
cell.  At  this  initial  stage  of  liberation  the  concentrated  heat  must  reach 
what  would  appear  to  us  as  an  enormous  temperature.  From  these  points 
it  rapidly  radiates,  to  be  distributed  as  lower  and  lower  temperatures 
to  the  various  parts  of  the  tissue  and  body. 

This  heat  is  produced  under  two  principal  circumstances.  First, 
as  a  step  in  the  process  of  generating  motion  or  as  a  by-product  in  the  other 
physiological  processes.  This  has  been  touched  upon  in  discussing  mo- 
tion. Second,  it  is  probably  produced  specifically  and  for  the  purpose 
of  maintaining  a  body  temperature. 

The  protoplasm  of  the  animal  body  would  not  operate  or  live  at  the 
absolute  zero  or  at  any  temperature  between  that  and  the  freezing  point. 
In  most  of  the  lower  animals  the  required  temperature  is  attained  by 
living  in  a  climate  whose  air  and  water  furnish  the  heat.  The  organisms 
which  live  and  are  active  in  Arctic  seas  and  on  the  sea  bottom  at  great 
depths  exist  normally  in  a  temperature  that  is  but  little  above  the  freez- 
ing point.  Others  require  more  heat,  and  while  they  will  not  die  for  a 
while  at  or  somewhat  below  the  freezing  point,  they  cannot  live  per- 
manently unless  in  a  temperature  considerably  above  this.  Therefore 
they  seek  a  warmer  climate  or  the  rays  of  the  sun  on  a  rock  at  noonday. 
Countless  forms,  especially  such  as  insects  and  reptiles,  live  through  the 
cold  of  winter  or  of  the  tropic  night,  in  the  high  altitudes,  and  only  come 
out  and  are  active  during  the  summer  time  or  the  heat  of  midday. 

141 


142  HISTOLOGY 

Other  animals  are  able  to  produce  a  considerable  amount  of  heat  by 
their  activity.  A  mackerel  is  said  to  raise  its  body  temperature  eight 
degrees  above  that  of  the  water  by  its  vigorous  swimming.  In  rest,  how- 
ever, it  returns  almost  to  the  temperature  of  the  water. 

Among  one  group  of  animals  only  is  the  function  found  of  maintain- 
ing a  temperature  constantly  above  and  independent  of  that  of  their 
surroundings.  These  are  the  vertebrate  animals,  and  only  two  divisions 
of  these,  the  birds  and  the  mammals,  do  this.  In  man  the  temperature 
is  about  thirty-eight  degrees,  while  in  some  birds  it  is  constantly  as  high 
as  forty-four  degrees.  Not  only  are  these  temperatures  high,  but  they  are 
constant  within  very  small  limits.  To  secure  this  constancy  there  must 
be  means  of  producing  more  heat  when  it  is  too  low,  of  lowering  it 
should  it  get  too  high,  and  of  properly  distributing  it  as  well  as  retaining 
it  in  the  body  against  radiation. 

Heat-production  must  be  stimulated  by  its  need,  by  nerves  which, 
when  the  temperature  gets  too  low,  automatically  cause  a  greater  produc- 
tion and  oxidization  of  thermochondria.  Also  muscular  exercise  liber- 
ates much  heat  due  to  the  myochondria  in  their  work  of  heating  the 
myo-fibrils  to  make  them  absorb  water  and  contract. 

This  heat  is  distributed  from  its  points  of  generation  by  radiation  and 
by  the  circulation  of  the  blood.  When  the  body  becomes  too  warm, 
heat  is  removed  by  the  evaporation  of  fluids  on  the  body  surfaces.  Sweat, 
on  the  outer  body  surface  in  the  horse  and  man,  and  other  fluids  in  the 
throats  of  animals  like  the  dog  and  the  common  domestic  fowl,  which 
"pant"  when  too  warm,  are  the  fluids  used.  The  extremities  of  these 
animals,  as  well  as  the  surfaces,  are  sometimes  much  below  the  body 
mass  in  temperature. 

There  is  but  little  histology  to  exhibit  concerning  this  function, 
although  there  are  tissues  more  or  less  set  aside  to  perform  it.  As 
mentioned  before,  the  muscles  probably  produce  most  of  the  heat,  the 
blood  distributes  it,  and  the  surfaces  of  the  body  release  it  and  lower  the 
temperature.  Any  accident  or  pathological  condition  in  these  may 
cause  irregularity  in  the  temperature,  sometimes  to  the  point  of  killing 
the  organism  by  a  reduced  or  excessive  amount  of  heat. 


LITERATURE 

VERWORN,  M.     "  General  Physiology." 
FOSTER,  M.     "  Physiology." 


CHAPTER  XII 
TISSUES   OF  CIRCULATION:    GENERAL  CONSIDERATIONS 

WE  have  seen  that  the  body  of  an  organism,  when  it  is  not  a  single 
cell,  is  a  mass  of  cells  closely  applied  to  each  other  in  a  manner  designed 
to  shut  out  most  outer  conditions  and  all  foreign  materials  except  such 
as  may  be  admitted  by  design.  We  also  know  that  all  of  the  cells  of  this 
body,  while  living,  must  be  taking  in  the  materials  used  to  sustain  life, 
while  at  the  same  time  they  are  throwing  out  and  discarding  certain  other 
materials  that  are  no  longer  wanted.  The  first  of  these  materials,  which 
we  may  call  the  food  materials,  must  be  brought  into  the  body  from  the 
outside.  They  are  taken  in,  through  or  between  the  cells  that  cover  the 
body  surface,  or  some  particular  part  of  this  surface.  This  is  very  evident, 
as  they  could  not  enter  in  any  other  way.  And  it  is  also  true  that  these 
surface  cells  must  act  as  the  medium  through  which  the  waste  matter 
spoken  of  above  is  cast  out.  This  thought  serves  to  impress  us  with  the 
importance  of  surface  as  a  factor  in  the  operations  of  the  animal  body 
and  of  the  constant  transfer  of  material  that  must  go  on  in  all  its  parts. 

More  important  for  present  discussion,  but  not  to  be  separated  from 
the  above  ideas,  is  the  fact  that  the  materials,  once  inside  the  body,  must 
be  passed  from  cell  to  cell,  must  be  distributed  and  must  be  gathered. 
This  means  that  they  are  constantly  transported.  Such  transportation 
in  its  simplest  form  is  a  physiological  process  of  the  cytoplasm  or  a  physi- 
cal process  of  osmosis  or  both.  Lastly,  it  may  happen  that  the  materials 
pass  between  the  cells  instead  of  through  them,  and  they  may  be  assisted 
on  their  way  by  fluids  that  carry  them  in  solution  or  otherwise.  Such  a 
system  of  passages  between  the  cells  constitutes  a  circulatory  system 
for  the  distribution  of  materials,  and  the  fluid  used  to  carry  the 'matter 
is  called  a  circulatory  fluid  or  blood. 

It  is  true  that  if  the  body  is  so  small  that  the  materials  can  be  easily 
passed  from  cell  to  cell  through  its  mass,  that  there  is  no  need  for  a  circu- 
latory system.  And  when  the  mass  of  the  body  becomes  too  great  for 
an  effective  distribution  from  the  outer  surface,  it  is  still  possible  to  both 
increase  the  amount  of  surface  and  to  make  the  distribution  effective  by 
a  series  of  invaginations. 

In  order  to  understand  the  comparative  value  of  such  a  method  and 
of  an  internal  transporting  or  circulatory  system,  we  shall  examine  a 

143 


144  HISTOLOGY 

series  of  imaginary  cellular  bodies  in  which  the  surface  is  supposed  to  be 
everywhere  equal  in  its  power  to  transfer  a  given  amount  of  material 
in  a  given  length  of  time,  and  in  which  this  material  is  likewise  passed  by 
physiological  processes  from  cell  to  cell  inside  the  body.  Each  cell,  in- 
cluding the  outer  cells,  is  supposed  to  use  what  food  it  needs  upon 
receiving  it,  and  to  pass  the  rest  along  equally  to  all  of  its  neighbors. 

Let  us  suppose,  for  discussion,  that  a  body  is  the  simplest  kind  of  a 
mass,  a  cube,  or  better  still  a  sphere  (Fig.  130,  4).  It  can  then  be  seen 
that  a  cell  situated  in  the  center  of  this  sphere  can  only  receive  food 


FIG.  1 30,  A,  B,  and  C.  —  A ,  body  of  cells  small  enough  to  secure  necessary  exchanges  through  sur- 
face cells.  B,  a  body  of  cells  too  large  to  do  the  same.  C,  same  sized  mass  of  cells  as  in 
B,  but  with  surface  increased  by  invagination  until  sufficient  to  work  necessary  exchanges. 

materials  from  the  surface  of  the  body  after  this  food  has  passed  through 
every  cell  that  lies  between  it  and  the  body  surface.  Suppose  further 
that,  in  this  spherical  body,  only  enough  material  to  supply  three  rows 
of  cells  can  be  passed  through  the  outer  row  or  layer  of  cells  (in  A) 
besides  what  it  requires  for  its  own  use,  and  that  in  this  case  the  organism 
is  just  able  to  live. 

If  now  we  suppose  that  the  organism  is  larger  (Fig.  130,  B),  to  have, 
for  instance,  six  rows  of  cells  from  center  to  surface,  and,  remembering 
that  the  surface  cells  are  capable  of  supplying  only  three  other  rows  of 
cells  with  food,  it  follows  that  in  this  case  they  have  more  than  they  can 
do,  and  the  inner  cells  must  perish  both  from  lack  of  food  and  from  an 
inability  to  get  rid  fast  enough  of  the  waste  substances  that  are  poisonous 
to  them.  The  inner  cells  would  die  first,  as  is  exemplified  in  the  case  of  the 
cancerous  growth  that  breaks  down  in  its  interior  when  it  has  reached  a 
certain  size  and  has  not  developed  a  circulation. 

Two  changes  in  the  structure  of  the  body  are  possible  that  would 


CIRCULATORY   TISSUES,    GENERAL 


145 


serve  to  remedy  this  condition.  They  have  already  been  mentioned: 
First,  a  series  of  one  or  more  invaginations  to  increase  the  surface  and  at 
the  same  time  to  bring  it  nearer  to  the  cells  to  be  supplied.  Second,  some 
system  of  circulation  of  a  fluid  in  the  body  to  carry  the  materials  more 
easily  to  their  destinations. 

Figure  130,  C,  shows  a  body  in  which  the  invagination  of  two  points 
on  its  surface  has  supplied  the  need,  and  the  materials  can  be  sufficiently 
widely  distributed.  In  Figure  131,  D,  the  same  principle  is  applied  to  a 


FIG.  131,  D.  —  Diagram  of  a  still  larger  body  with  extensive  system  of  invaginations.    Sufficient 
surface  but  no  adaptability. 


body  of  much  greater  dimensions,  and  it  is  successful  as  a  mere  distribu- 
ter. But  it  can  now  be  seen  that  in  a  body  of  any  size  the  complexity 
would  become  very  great.  Perhaps  of  greater  importance  than  this  is 
the  fact  that  each  invagination  would  have  to  do  exactly  the  same  kind 
of  work  that  every  other  one  did,  because  there  would  be  no  way  for  the 
products  of  differentiated  invaginations  to  be  exchanged.  An  example 
of  an  animal  built  on  exactly  such  lines  can  be  found  in  the  sponge,  and 
such  a  condition  constitutes  in  itself  a  form  of  low  specialization  from 
which  there  is  no  possible  advance.  Evidently  invagination  alone  is 
not  a  change  of  structure  by  which  much  can  be  accomplished. 

Turning  to  the  second  of  the  two  changes  of  structure  that  were 
recognized  as  solutions  of  our  primary  difficulty,  we  must  examine  a  body 


146 


JfJS'JOLOGY 


of  the  same  size  as  the  one  which  we  have  just  been  examining,  in  which 
no  invagination  has  been  performed,  but  in  which  certain  of  the  inner 
cells  have  been  separated  to  form  a  series  of  communicating  channels 
containing  a  fluid  that  can  carry  the  materials  of  which  we  are  speaking 
and  distribute  or  collect  them.  Figure  132,  E,  shows  such  a  case  dia- 
grammatically.  This  structure  has  solved  the  single  problem  of  distribu- 
tion and  has  relieved  the  cells  of  the  burden  of  passing  materials  for  such 
long  distances  through  the  body.  But  it  has  not  created  any  more  sur- 


T~<- — i      i^ — •— 

FIG.  132,  E.  —  Diagram  of  a  large  mass  of  cells  with  perfect  circulatory  system  but  insufficient 

surface. 


face  for  the  transference  of  materials,  and,  as  the  original  surface  was  not 
sufficient  for  this  purpose,  it  can  be  seen  that  the  interior  cells  of  this 
organism  also  must  die.  The  only  difference  is  that  the  inner  cells  will 
all  perish  together  instead  of  the  innermost  first. 

Let  us  now  consider  a  case  where  a  system  of  imaginations  to  increase 
surface  is  developed  in  connection  with  a  system  of  internal  circulation 
to  properly  distribute  the  materials  elaborated  by  the  surfaces  of  the  in- 
vaginations.  Figure  133,  F,  shows  such  a  condition,  and  it  needs  but  a 
minute  of  thought  to  see  that  the  objectionable  features  of  either  in- 
vagination alone,  as  in  Figure  131,!},  or  of  circulation  alone,  as  in  Figure 
132,  E,  are  solved  by  the  combination  of  the  two.  The  lack  of  surface 
in  Figure  132,  E,  is  provided  for  by  the  invaginations  that  were  found 


CIRCULATORY   TISSUES,    GENERAL  Itf 

alone  in  Figure  131,  D,  but  are  combined  with  the  circulation  in  Figure 
133,  F.  The  clumsiness  and  especially  the  lack  of  possible  differentia- 
tion of  function  in  the  separate  invaginations  in  Figure  131,  D,  are  done 
away  with  by  the  circulation  of  Figure  132,  £,  as  applied  in  Figure  133, 
F.  One  should  now  use  the  imagination  and  realize  the  fact  that  this 
arrangement  can  be  enlarged  to  almost  any  degree  of  complexity  and 
adapted  to  almost  any  need.  A  large  number  of  different  sorts  of  work 
can  be  done  in  the  different  invaginated  regions,  and  by  the  extension 


FIG.  133,  F.  — Large  mass  of  cells  with  perfect  circulatory  system  and  sufficient  surface. 

and  formation  of  new  loops  and  branches  of  the  circulatory  channels  the 
products  of  these  activities  can  be  carried  to  any  part  of  the  organism 
where  they  may  be  needed,  or,  if  not  needed,  to  any  part  of  the  body 
where  it  is  possible  to  get  rid  of  them. 

The  Figures  130  to  133  are  diagrammatically  correct  as  to  relations  of 
surface  to  bulk  and  to  thickness  of  cell  layers.  In  each  case  the  source 
of  surface  supply  is  indicated  by  different  formal  shadings  of  the  cells. 
The  blood  channels  shown  in  Figures  132  and  133  are  the  same,  and 
the  invaginations  are  the  same  in  Figure  131,  D,  and  Figure  133,  jp, 
with  the  exception  that  the  smallest  invagination  in  Figure  131,  D,  is 
omitted  from  Figure  133,  F,  to  indicate  the  possibility  of  a  region  re- 
mote from  the  surface,  in  a  body  supplied  with  a  circulation.  To  pre- 
serve the  same  value  of  surface,  however,  the  place  of  this  smallest 


148  HISTOLOGY 

invagination  is  taken  in  133,  F,  by  small  additional  extensions  from 
the  three  remaining  invaginations. 

Such  is  the  fundamental  idea  of  the  circulatory  system  and  its  rela- 
tions to  the  system  of  invaginations  of  the  original  surface  of  the 
animal  body.  We  shall  now  consider  it  apart  from  its  relations  to 
the  differentiation  of  the  surface  functions  and  merely  as  an  agent  of 
distribution. 

The  circulatory  channels  appear  in  their  simplest  form,  both  taxi- 
nomically  and  ontogenetically,  as  one  or  more  irregular  spaces  in  the 
inner  or  mesodermal  regions  of  the  animal  body.  These  spaces  have 
no  boundaries  other  than  the  mesodermal  cells  among  which  they  lie, 
and  some  of  these  cells  become  detached  and  float  freely  in  the  lymph 
fluid  that  fills  the  lumen.  Later,  most  of  these  mesodermal  cells  be- 
come specialized  into  connective-tissue  cells,  while  many  of  them,  in 
the  neighborhood  of  the  blood  vessels,  become  modified  to  form 
the  specialized  walls  of  these  spaces.  Those  in  the  fluid  of  the  lumen 
may  become  blood  corpuscles  or  they  may  also  join  in  forming  the 
walls.  At  such  a  period  the  circulatory  space  is  not  to  be  distin- 
guished from  other  body  cavities  that  may  afterwards  become  separate 
or  partly  separate  from  it. 

A  low  form  of  a  specialization  of  this  primary  circulatory  space  is  its 
enlargement  into  one  or  more  long,  continuous  cavities  extending  the 
length  of  the  body  and  into  the  limb  or  appendages.  The  blood  is  driven 
about  in  this  set  of  channels  by  the  movements  of  the  general  muscula- 
ture of  the  body,  and  in  some  cases  it  exhibits  a  special  rhythmic  move- 
ment, passing  first  toward  the  anterior  part  of  the  body  and  then  in  the 
opposite  direction.  This  cavity  also,  since  it  occupies  the  greater  part  of 
the  body  and  contains  the  organs,  must  be  looked  upon  as  the  body-cavity 
or  ccelom. 

This  development,  in  some  animals,  consists  of  the  growth  of  a  part 
of  the  early  ccelomic  cavity  into  a  long,  tube-like  channel  with  many 
branches  and  a  more  or  less  definite  wall.  In  this  tube,  which  may  be  a 
closed  circuit  or  only  a  partial  circuit,  the  blood  acquires  a  continuous 
movement,  being  pumped  through  the  system  usually  in  one  direction. 
At  this  point  in  its  history  the  blood-channel  system  is  usually  separated 
more  or  less  completely  from  a  remaining  portion  of  the  ccelom,  which 
we  shall  call  the  body-cavity.  Many  other  differentiations  and  separa- 
tions from  these  cavities  occur,  as  the  cavities  in  connection  with  the  hearts 
and  the  nephridial  organs,  the  secondary  reproductive  organs,  and  the 
lymphatic  system  and  its  various  modifications.  In  the  higher  animals 
the  blood-channel  system  arises  de  novo  as  a  series  of  clefts  in  mesodermal 
tissue.  There  may  be  different  regions  of  a  circulatory  system  which  are 
separate  and  contain  different  kinds  of  circulating  media. 


CIRCULATORY  CHANNELS  149 


LITERATURE 

SCHNEIDER,  K.  C.     "  Lehrbuch  der  Algem.  Histologie,"  Jena,  1898. 
FOSTER,  M.     "Text-Book  of  Physiology." 


TISSUES  OF  CIRCULATION:   THE  HISTOLOGY  OF  THE  CHANNELS 

The  main  blood-channel  system  itself  has  many  differentiated  regions. 
The  region  of  thin-walled  capillaries  and  lacunae,  the  strong-walled  con- 
ducting vessels,  the  blood-forming  regions,  and  the  muscular  pumping 
stations  or  hearts  are  the  chief  grouping  of  these  organs  which  must  be 
treated  of  in  more  detail  in  the  seminar  part  of  this  section.  Most  im- 
portant, or  rather  most  specific  of  these  portions,  are  the  capillaries  and 
lacuna,  for  it  is  here  that  the  real  work  of  the  blood  is  accomplished,  the 
exchange  of  materials  with  the  tissues.  This  region  will  be  spoken  of 
as  the  periphery.  Here  the  walls  of  the  vessels  are  thinnest  or  even  ap- 
parently wanting.  In  this  case  the  connective-tissue  cells  that  surround 
the  channel,  while  not  differentiated  into  definite  channel  walls,  act  in 
that  capacity,  so  that  we  cannot  say  that  retaining  walls  are  altogether 
absent.  The  vessels  of  the  periphery  have  in  all  cases  a  larger  total  cross 
section  than  any  other  total  cross  section  in  the  circuit.  This  results 
in  the  surface  of  contact  between  blood  and  tissue  being  large  enough 
to  effect  necessary  exchanges  of  materials  as  well  as  making  the  current 
slower  to  give  requisite  time  for  such  exchanges. 

The  smaller  but  more  numerous  branches  of  the  periphery  unite  to 
form  larger  channels  that  serve  to  conduct  the  blood  to  other  portions 
of  the  periphery,  or  to  and  from  the  central  pumping  stations,  or  to  the 
blood  glands.  These  vessels,  the  veins,  together  with  the  vessels  carrying 
blood  back  to  the  periphery,  the  arteries,  act  as  the  long-distance  carriers 
of  the  circulatory  system,  and  their  walls  are  usually  very  strongly  con- 
structed. 

The  pumping  region  comprises  one  or  more  parts  of  the  larger 
channel  or  channels  that  have  acquired  the  power  of  rhythmic  contrac- 
tion. Sometimes  this  region  occupies  a  considerable  extent  of  the  larger 
vessels.  At  other  times  it  is  found  in  a  more  specialized  form,  occupying 
only  a  short  section  of  the  tube,  but  very  intensely  developed.  Such  an 
organ  is  known  as  a  heart.  Both  of  the  preceding  conditions  may  be  found 
together,  as  they  are  in  the  squid  and  other  cephalopod  mollusks,  where 
there  are  three  or  five  separate  hearts,  and  in  addition  the  larger  part  of  the 
arteries  are  also  constantly  engaged  in  driving  the  blood  on  its  course  by 
wave-like  pulsations. 

Other  regions  of  the  blood -channel  system  are  found  in  which  the 
walls  are  differentiated  and  in  which  the  blood  moves  but  slowlv  and  some- 


I5O  HISTOLOGY 

times  almost  comes  to  rest.  These  form  the  so-called  blood  glands,  and 
in  them  the  blood  is  renovated  by  the  removal  of  some  of  its  old  parts  or 
the  addition  of  other  new  ones  or  both.  There  are  several  kinds  of  these 
organs  and  they  will  be  treated  of  later. 

Owing  to  the  homogeneous  histological  structure  of  the  circulatory 
organs  in  the  various  groups  of  animals,  we  shall  study  the  detail  of  these 
organs  by  going  through  the  individual  system  of  several  typical  forms. 
It  must  be  held  in  mind  that  the  walls  of  these  organs  show  a  strong 
analogy  based  on  the  physiological  (which  are  here  mechanical)  needs  of 
the  vessels.  The  blood  fluid  must  be  confined  to  the  channels,  and  this  is 
usually  done  by  the  single  inner  layer  of  cells,  the  intima.  In  some  forms 
the  intima  is  formed,  not  by  the  cells  themselves,  but  by  a  cuticle  which  is 
the  product  of  these  cells  (see  the  paragraphs  on  the  lobster  and  Imperia- 
lis  larva).  The  intima  may  alone  confine  the  blood  stream,  or  if  the 
pressure  is  too  great,  it  may  be  reenforced  by  the  connective-tissue  cells 
that  immediately  surround  it.  These  cells  develop  their  connective  tissue 
as  fibrils  or  plates  or  webs  with  which  they  bind  and  hold  the  vessel  in- 
tact when  the  blood  presses  on  its  walls.  Again,  these  primitive  meso- 
derm  cells  may  develop  into  muscle  cells  that  surround  the  channel  and 
by  their  contractile  strength  cause  it  to  pulsate  and  drive  the  blood  on  its 
course.  The  arrangement  of  these  three  classes  of  tissues  to  form  the 
wall  of  the  vessel  falls,  naturally,  into  layers,  the  so-called  coats  of 
the  blood  vessels.  Each  kind  of  coat  usually  has  a  particular  posi- 
tion with  reference  to  the  lumen.  This  position,  however,  is  some- 
times changed  in  the  several  groups  for  no  apparent  reason. 

All  these  cells  and  the  tissues  that  they  form  were  probably  not  cells 
that  were  bound  in  the  course  of  their  development  to  become  so  special- 
ized, but,  as  far  as  can  be  told,  they  were  such  of  the  connective-tissue 
cells  as  happened  to  be  in  the  course  of  the  developing  blood  channel  as 
it  pushed  its  way  among  them,  and  were  developed  in  response  to  the 
needs  of  the  vessels.  Any  of  these  connective-tissue  elements  would 
probably  do  the  same  if  the  blood  vessel  came  their  way,  especially  in 
the  embryonic  stages  of  the  organism.  This  view  is  open  to  debate, 
however,  until  observation  has  brought  proof. 

We  shall  study  the  walls  of  the  blood  vessels  in  a  few  typical  forms 
to  see  what  variation  is  found  among  them  from  a  histological  point  of 
view.  Some  Turbellarian  as  a  primitive  form;  the  wormCerebratulus, 
the  earthworm  Allolobophora,  the  mollusk  Unio,  the  mollusk  Octopus, 
the  crustacean  Homarus,  and  an  insect,  Imperialist  Amphioxus  with 
reference  to  a  Tunicate,  and  the  Vertebrate,  man,  with  reference  to  a 
salamander,  will  cover  the  ground  satisfactorily.  Forms  lower  than  the 
Nemerteans  seldom  possess  a  circulatory  system. 

The  internal  tissue  of  a  Turbellarian  worm  is  a  loose  aggregate  of 


CIRCULATORY  CHANNELS 


several  kinds  of  weakly  differentiated  cells,  known  as  a  parenchyme. 
These  cells  do  not  touch  each  other  at  all  points,  but  are  connected  by 
strands,  and  in  consequence  there  may  be  easily  seen  between  them  a 
great  many  spaces,  known  as  the  intercellular  spaces,  which  are  united 
into  a  large  connecting  system  that  extends  throughout  the  body 
(Fig.  134). 

This  system  of  spaces  is  filled 
with  a  fluid  and  this  fluid  carries 
the  digested  food  materials,  the  oxy- 
gen supply  for  internal  cells,  the 
combustion  products,  and  in  every 
other  way  acts  as  a  simple  blood. 
This  is  the  undifferentiated  and  un- 
organized form  of  blood-vessel  sys- 
tem, and  a  sort  of  circulation  must 
inevitably  take  place  as  a  result  of 
the  ordinary  movements  of  the  ani- 
mal's body.  This  grade  of  structure 
is  to  be  seen  in  a  number  of  the 
lower  and  simpler  animal  forms  and 
sometimes  as  an  accessory  apparatus 
to  several  grades  of  complete  blood- 
channel  systems. 

Structure  of  the  Blood- Vessel  Walls  in  the  Nemertean  Worm,  Cere- 
bratulus.  — The  location  of  the  blood  vessels  is  a  morphological  matter. 
They  can  be  found  for  our  study  in  the  connective  tissue  around  the  di- 
gestive tract >  particularly  a  large  thick- walled  vessel  between  the  oesopha- 
gus and  the  rhynchoccel.  A  branch  of  this  that  runs  circularly  around 
the  digestive  tract  will  be  studied  in  longitudinal  sections  (Fig.  135). 
This  vessel  is  lined  with  an  endothelial  layer  of  cells  that  are  very  thin 
and  delicate  and  can  only  be  observed  to  advantage  in  well-hardened 
material.  Otherwise  one  is  liable  to  confound  their  nuclei  with  those  of 
the  blood  corpuscles.  When  the  vessel  is  contracted,  as  it  is  in  many 
fixations,  this  layer  is  thrown  into  longitudinal  folds,  and  the  nuclei 
usually  lie  in  that  part  of  the  fold  that  projects  into  the  lumen.  The 
individual  cells  are  elongate,  as  are  also  their  nuclei,  and  the  reticulum  of 
cytoplasm  has  longitudinal,  drawn-out  meshes  that  give  the  cell  body  a 
striated  appearance  in  the  section  we  are  examining. 

The  endothelium  rests  upon  a  very  weak  basement  membrane  of 
so  little  substance  that  it  seems  to  be  a  mere  boundary  in  the  thinner 
vessels,  but  of  appreciable  thickness  in  the  larger  arteries.  In  some  fixa- 
tions it  seems  to  be  radially  striated,  but  this  appearance  is  probably  due 
to  fine  longitudinal  folds  and  the  contact  of  the  endothelium.  Directly 


FIG.  134.  — •  Body  tissue  from  a  small  flat- 
worm  to  show  the  intercellular  clefts  (ch.) 
which  act  as  circulatory  channels.  X  700. 


152 


HISTOLOGY 


c.mus. 


outside  of  the  basement  membrane  comes  a  layer  of  circular  muscle 
fibers  that  are  remarkable  for  the  fact  that  no  nuclei  appear  in  their  sub- 
stances or  directly  among  them.  Closer  examination  will  show  that  these 
structures  are  not  fibers,  as  are  the  non-striated  elements  of  vertebrate 
smooth  muscle,  but  fibrils  or  small  groups  of  fibrils,  each  one  or  two  of 
which  belong  to  one  of  the  large  cells  that  lie  in  the  layer  of  tissue  just 
outside  of  them.  Each  one  of  these  structures  is  most  probably  a  group  of 
several  myo-fibrils,  and  not  a  fiber  in  the  sense  that  such  a  structure  is 

spoken  of  in  the 
vertebrate  ani- 
mal, for  in- 
stance. They 
lie  three  or  four 
thick  in  the 
larger  vessels 
and  in  a  de- 
creasing propor- 
tion  in  the 
smaller  chan- 
nels, until  in  the 
smallest  there 
are  none  to  be 
found.  The  cel- 
lular layer  found  outside  of  the  circular  muscle-fibrils  consists  of  the  cell 
bodies  to  which  the  muscle-fibrils  belong,  together  with  a  few  connective- 
tissue  cells  and  an  occasional  nerve  cell.  These  form  an  epithelial -like 
layer  arranged  radially  and  containing  elongated  nuclei.  When  the 
vessel  is  contracted,  the  cells  are  elongated  and  columnar  in  form,  and 
when  the  blood  distends  the  channel,  they  shorten  and  become  cubical. 

The  majority  of  the  vessels  possess  no  further  covering  except  the 
very  slight  amount  of  connective  tissue  found  around  and  among  the 
muscle  cells.  The  dorsal  vessel  and  larger  branches  possess,  in  addition 
to  the  tissues  already  described,  an  outer  layer  of  longitudinal  muscle 
fibers.  They  appear  in  transverse  sections  of  the  vessel  as  roughly  circu- 
lar sections  of  the  cylindrical  cytoplasmic  bodies  of  the  muscle  structures, 
each  containing  a  number  of  the  same  fibril  groups  that  we  have  observed 
in  the  other  muscles  of  this  animal.  A  single  nucleus  appears  in  each 
section  and  these  are  probably,  therefore,  syncytia.  In  our  longitudinal 
sections  this  relation  of  nuclei  and  muscle-fibrils  to  the  cell  is  not  so  plain, 
but  can  be  grasped  by  a  comparison  with  transverse  sections  in  the  same 
specimen. 

It  will  be  noticed  of  this  circulatory  system  that  it  is  simple  and  un- 
specialized  in  the  fact  that  all  its  parts  are  substantially  alike,  that  it  is 


FIG.  135.  — Longitudinal  section  of  part  of  the  wall  cf  a  blood  vessel  cf 
Cerebratuliis  lactatus.  int.,  intima,  a  layer  of  cells  with  a  very  delicate 
membrane;  c.mus.,  bundles  of  circular  muscle  fibers;  mus.c.,  cells  to 
which  these  muscle  bundles  belong;  l.mus.,  longitudinal  muscle  fibers. 


CIRCULATORY  CHANNELS  153 

muscular  and  therefore  contractile  for  the  greater  part  of  its  length,  and 
that  it  lacks  any  of  the  passive  connective-tissue  structures  found  so 
often  in  systems  that  have  a  central  pumping  station  or  heart.  This  is 
because  the  muscle  fibers  perform  the  function  that  such  connective-tissue 
cells  would  be  required  for,  at  the  same  time  acting  as  the  heart. 

The  Structure  of  the- Blood- Vessel  Walls  in  the  Earthworm,  Allolobo- 
phora.  —  The  earthworm  has  a  system  of  blood  channels  that  are  some- 
what harder  to  understand  than  those  of  Cerebratulus.  Owing  to  the 
delicacy  of  many  of  its  structures,  several  diverging  views  are  held  which 
cannot  be  fully  considered  here. 

As  occurs  in  all  blood  vessels,  the  walls  are  formed  by  layers.  The 
innermost  of  these  layers  is  much  questioned.  Those  who  consider  it  as 
an  existent  formative  layer  of  the  vessel  acknowledge  that  it  is  not  every- 
where present  in  the  blood-channel  system,  but  only  in  the  larger  vessels. 
It  is  described  as  composed  of  cells  with  flat,  small  nuclei,  and  the  cell 
bodies  form  a  very  thin  and,  at  parts,  incomplete  lining  of  the  vessel. 
The  cell  bodies  are  extended  in  the  axis  of  the  vessel,  and  it  is  not  possible 
to  define  the  lines  of  juncture  by  the  silver  method.  The  opponents  of 
this  idea  assert  that  these  cells  are  blood  cells  that  are  clinging  to  the 
arterial  walls  rather  than  parts  of  the  structure  of  the  wall.  We  shall 
consider  it  to  be  an  integral  part  of  the  vessel  in  consideration  of  the 
important  part  it  plays  in  the  larger  vessels  and  "  hearts  "  where  it  forms 
valves. 

Outside  of  this  layer,  and  found  throughout  the  channel  system  of 
which  it  forms  the  real  blood-retaining  boundary,  is  a  homogeneous 
membrane,  the  "  basement  membrane  "  or  intima.  This  is  a  clearly 
defined  and  denser  as  well  as  a  thicker  structure  than  the  intima  found 
in  the  blood  vessels  of  Cerebratulus.  When  the  vessel  is  contracted,  this 
intima  is  thrown  into  longitudinal  folds.  It  stains  red  with  Van  Gieson's 
picro-acid  fuchsine. 

Outside  again  of  the  intima  one  finds  two  or  three  distinct  types  of 
structure,  which  distinguish  the  several  kinds  of  vessels.  We  shall 
examine  first  the  thick  walls  of  one  of  the  several  semicircular  and  tubular 
"  hearts."  The  inner  endothelial  layer  is  very  thick  here  (Fig.  136). 
It  is  thrown  up  at  several  points  into  heavy  masses  which  oppose  each 
other  in  pairs  and  act  as  valves,  one  of  which  is  shown  in  the  figure. 

The  intima  is  very  thick  and  is  either  striated  or  folded  longitudinally 
or  is  covered  on  the  outside  by  longitudinal  muscle  fibers.  Outside  of 
this  layer  come  the  heavy,  plain,  smooth  muscle  fibers  of  the  circular  layer. 
They  are  irregularly  angular  in  section,  with  a  deeply  staining  central 
mass  (an  artifact),  and  their  cell  bodies  lie  outside  of  them  as  large,  well- 
developed  cells  with  large,  round  nuclei  that  are  only  seen  in  a  few  of  the 
cell  bodies  on  account  of  the  length  of  these  latter.  Outside  of  the  muscle- 


154 


HISTOLOGY 


us.   b. 


cell  bodies  is  a  layer  of  body-cavity  cells  which  surround  all  vessels 

that  pass  through  that  space. 

In  the  smaller  blood  vessels  the  place  of  the  muscle-cell  layer  is  taken 

by  a  layer  of  cells  called  the  "  wall  cells,"  which  partly  encircle  the 

vessel  and  are  contractile  in  the  arteries  and  non-contractile  in  the  veins. 

They  are  furnished  with 
fibrils  which  show,  accord- 
ing to  Schneider,  a  clear 
cross  striation  and  must 
therefore  be  considered  to 
be  myo-fibrils. 

The  Echinoderms  pos- 
sess a  Peculiar  Blood  and 
a  Very  Weakly  Developed 
System  of  Channels  to 
Carry  It.  — The  histology 
of  their  walls  is  extremely 
simple,  an  endothelium 
resting  upon  a  loose  con- 
nective tissue.  These  ves- 
sels widen  into  various 
lacunae  whose  walls  are 
similar. 

The  Mollusca  all  pos- 
sess very  definite  blood 
channels  which  are  differ- 
entiated into  well-defined 
regions.  An  artery  (foot 
artery)  of  Anodonta  shows 
the  following  structure  in 
its  walls  (Fig.  137).  An 
endothelium  of  irregular, 
longitudinally  extended 
cells  forms  the  innermost 

layer.     Some  of  these  cells  are  supposed  to  become  detached  and  form 

the  blood  and  lymph  cells.    They  are  absent  in  some  of  the  largest 

blood  vessels. 

This  epithelium  rests  on  a  basement  membrane  of  uniform  thickness, 

which  forms  a  real  blood-retaining  boundary.    This  membrane  must  be 

considered  as  a  surface  of  the  connective-tissue  cells  which  are  found 

outside  of  it. 

Lying   in  the  connective  tissue   and   almost   forming  a  boundary 

between  it  and  the  basement  membrane  are  a  number  of  single  muscle 


FIG.  136.  —  Longitudinal  section  of  part  of  the  wall  of  a 
"heart"  in  the  wire-  worm,  Allolobophora.  int.,  intima, 
which  is  enlarged  into  a  valve  v. ;  cu.,  cuticular  base  of 
intima;  mus.c.,  circular  muscle  cells  whose  fibril 
bundles  show  in  transection  at  mus.b. 


CIRCULATORY  CHANNELS 


155 


fibers  which  surround  the  vessel  as  a  circular  layer.  They  are  smooth, 
and  the  nucleus  lies  on  one  side  in  the  main  cell  body.  The  largest 
arteries  show  a  longitudinal  layer  of  muscle  fibers  which,  near  the  heart, 
become  irregular  in  arrangement  as  they  also  are  in  that  organ. 

In  the  Cephalopod  mollusks  is  found  a  series  of  blood  vessels  that 
are  much  the  same  in  structure  as  those  of  Unio  or  of  any  other  mollusk. 
A  point  of  importance  here  is  the  great  development  of  muscle  in  all  of 
the  larger  channels,  veins  and  arteries  alike.  These  vessels  are  active 
as  pumping  agents,  operating  without  the 
aid  of  many  valves.  Their  powerful  circu- 
lar muscle  layer  sends  waves  of  contrac- 
tion along  the  vessel.  These  waves  are  so 
strong  that  they  close  the  vessel  entirely  and 
drive  the  blood  before  them. 

Figure  138  shows  a  large  mantle  artery 
from  a  common  Florida  octopus.  The 
inner  layer  consists  of  a  very  thin  endothe- 
lium  lying  on  a  thick,  well-developed  mem- 
brane. This  membrane,  from  its  position 
under  a  layer  of  surface  cells,  is  a  base- 
ment membrane.  But  also,  its  compara- 
tively great  thickness  reminds  one  of  the 
elastic  membrane  to  be  described  later  in 
a  human  artery.  It  probably  is  greatly 
strengthened  and  added  to  in  substance  by 
the  connective  tissue  on  which  it  lies.  It 
lies  in  longitudinal  folds  in  all  but  the 
fullest  expanded  arteries,  and  it  is  possibly,  but  not  necessarily,  non- 
elastic. 

The  connective  tissue  lying  outside  of  the  membrane  is  very  sparse 
in  our  subject,  the  octopus,  and  can  be  seen  to  better  advantage  in  the 
contracted  mantle  artery  of  a  squid.  Its  structure  has  no  great  signifi- 
cance other  than  its  function  as  a  loose  and  movable  connecting  medium 
between  the  layers. 

The  muscle  cells  form  a  thick,  powerful  layer  of  circular  fibers  em- 
bedded in  a  connective  tissue  which  holds  them  together  and  at  the  same 
time  keeps  them  some  distance  apart.  The  individual  muscle  fibers  are 
comparatively  short,  bluntly  spindle-shaped,  and  reach,  in  the  average 
example,  about  one  tenth  of  the  distance  around  the  circumference  of  the 
artery. 

The  fine,  sharp  strands  or  fibrils  of  connective  substance  which  bind 
these  fibers  together  are  loosely  arranged  between  them  except  on  their 
blunt  ends.  Here,  these  fibrils  reach  radially  from  the  end  of  the 


mus.fi.b. \ 


FIG.  137. — Part  of  a  transaction 
of  a  foot  artery  of  Anodonta, 
int.,  intima  of  endothelial  cells 
on  a  basal  membrane  of  weak 
development;  mus.fi.b.,  muscle 
fibril  bundles  (fibers);  conn.  I.e., 
connective-tissue  cell.  (After 
SCHNEIDER.) 


1 56 


HISTOLOGY 


mus.n. 


muscle  fiber  to  points  of  attachment  in  all  directions,  especially  on  the 
sides  of  adjacent  fibers.  They  do  not  appear  on  all  fiber  ends  in 
the  figure  because  many  are  but  apparent  ends,  due  to  oblique  cutting. 
Considering  the  connecting  fibrils  found  on  the  surface  of  smooth  muscle 
cells  in  other  animals,  it  is  probable  that  these  fibrils  and  much  of  the 
other  fibrillar  material  lying  between  the  muscle  cells  are  products  of 

those  cells  and 
not  of  separate 
connective-tissue 
units. 

The  muscle 
nuclei  are  very 
large  and  lie  on 
the  sides  of  the 
fibers.  In  this 
respect  they 
differ  from  the 
more  familiar 
smooth  muscle 
fibers  of  mam- 
mals. They  are 
oval  in  form 
and,  like  other 
muscle  nuclei, 
have  but  little 
chromatin  and  a 
small  nucleolus. 
Outside  of 
the  thick  layer 
of  circular  mus- 
cle comes  the  layer  of  longitudinal  muscle.  It  is  separated  from  the 
circular  layer  by  a  region  of  connective  tissue.  Where  the  vessel 
traverses  a  cavity,  either  internal  or  external,  the  characteristic  lining 
of  that  cavity  is  reflected  over  it  and  forms  a  layer  external  to  the 
longitudinal  muscle.  The  smaller  vessels  lose  the  outer  layers  until,  in 
the  capillaries,  their  only  wall  is  a  single  endothelium  showing  evi- 
dences of  its  connective-tissue  origin  (Fig.  139). 

The  blood  vessels  of  the  Arthropoda  are  peculiar  in  several  ways. 
Perhaps  their  most  marked  characteristic  is  a  single  connective-tissue 
epithelium  that  forms  their  chief  wall.  Also  the  fact  that  they  are  lined 
with  an  inner  cuticle.  We  shall  study  one  example  in  a  crustacean  and 
another  in  an  insect. 

The  blood-vessel  system  of  the  lobster  is  a  good  one  to  examine,  and 


»•— 


FIG.  138. — Transaction  of  part  of  an  artery  wall  of  Octopus,  int., 
intima,  consisting  of  a  thin  layer  of  cells  lying  on  a  thick  homoge- 
neous membrane.  A  muscle  layer  consisting  of  circular  cells 
whose  stout  muscle-fibril  bundles  (fibers)  show  the  peculiar,  irregu- 
lar striation  found  in  mollusks  ;  mus.n.,  muscle  cell  nucleus  ;  conn, 
fi.,  connective  fibrils  at  end  of  a  fiber  ;  l.mus.,  longitudinal  muscle 
fibers.  X  700. 


CIRCULATORY  CHANNELS 


157 


FIG.  139. — Section  of  a  small  capillary  of  Octopus, 
containing  the  slightly  shrunken  blood  content  in 
which  two  blood  cells  appear.  X  580. 


consists  of  a  well-defined  pulsating  region  or  heart,  large  carrying  vessels, 
capillary  and  lacunar  peripheral  parts  as  well  as  the  easily  studied  blood 
glands.  We  shall  begin  with  a  study  of  the  lacunar  spaces  (see  Fig.  67). 
These  can  be  found  in  many  parts  of  the  body,  and  serve  to  exhibit  a  case 
where  the  blood  vessel  is  shown  in  its  true  light,  as  a  cleft  between  masses 
of  the  connective  tissue 
whose  cells  constitute  its 
walls. 

The  Leidig's  connective- 
tissue  cells  which  border  on 
these  lacunae  exhibit  hardly 
a  trace  of  differentiation. 
The  single  peculiarity  which  ^  x  "--' 

they  show  seems  to  be  a 
slight  cuticular  formation  on 
the  surface  which  they 
present  to  the  blood.  In 
the  arteries  which  carry  blood  into  the  sinuses,  a  difference  exists 
which  represents  the  greatest  differentiation  of  the  connective-tissue 
walls  (Fig.  140).  The  contiguous  lining  cells  are  smaller  and  have 
acquired  a  proximo-distal  striation,  which  is  thus  at  right  angles  to 

the  vessel's  surface.  They 
also  secrete  a  cuticle  of 
some  thickness,  and  this 
cuticle  most  probably  is 
elastic  during  life.  It  is 
irregular,  in  that  it  con- 
tains thickened  ribs  which 
run  in  the  direction  of  the 
vessel's  course. 

Those  cells  lying  just 
outside  of  the  epithelium 

FIG.  140.  —  Part  of  a  transection  of  the  wall  of  a  lobster's      layer    are    also    differenti- 
artery;  conn.t.l.,  connective-tissue  layer;  c.l.,  cell  lining          -    j        fUm.    V,o«o    rlpirpl 
on  which  appears  the  cuticle  (cu.).     X  700.  ate(1-          ^    naVC     aeVC1" 

.   oped    a    great    profusion 

of  circular  connective-tissue  fibrils  that  hide  the  cytoplasm  and  among 
which  the  nuclei  are,  at  first,  difficult  to  see.  These  fibrils  are  either 
elastic  or  their  curled  and  twisted  arrangement  permits  of  a  spring-like 
elasticity,  even  if  they  are  not  elastic.  Some  of  the  veins  show  condi- 
tions which  are  intermediate  between  this  and  the  sinus.  A  few  of 
them,  in  particular  positions,  show  a  flat  epithelium-like  arrangement 
of  the  lining  cells  and  a  development  of  very  fine  fibrils  directly  in  this 
first  layer  (see  Fig.  146). 


conn.  1. 1. 


158 


HISTOLOGY 


The  gill  capillaries  of  Amphitrite  are,  apparently,  one  of  the  excep- 
tional cases  where  a  blood  vessel  loses  its  connective- tissue  covering 
entirely  and  allows  the  blood  to  flow  directly  among  and  between  the 
cells  of  an  ectodermal  epithelium.  It  is  still  possible,  however,  that  a 
delicate  layer  of  mesodermic  cytoplasm,  too  thin  to  have  been  heretofore 
detected,  follows  them  as  their  true  covering  (see  Fig.  293). 

The  insects  have  a  circulatory  apparatus  which,  considering  the  high 
specialization  of  the  group,  is  remarkably  simple.  It  consists  of  one 
very  large  vessel,  extending  for  some  distance 
in  the  median  line  of  the  back.  It  is  mus- 
cular and  serves  to  carry  the  blood  from  one 
end  of  the  body  to  the  other,  and,  after  a 
very  poor  distribution  by  means  of  a  few 
short  vessels  in  its  anterior  region,  it  receives 
it  again,  as  it  niters  its  way  back,,  through 
large  sinuses  and  spaces.  It  then  pumps  it 
forward  once  more.  Separate  pumping  or- 
gans in  the  limbs  suffice  to  carry  a  stream 
into  the  smaller  extremities. 

The  dorsal  heart  of  a  moth  larva,  Im- 
perialis,  is  typical  and  consists  of  three  layers, 
or  five  if  we  consider  the  outer  and  inner 
layers  double  (Fig.  141).  Each  of  these  two 
last  layers  consist  of  a  sheet  of  flat  cells 
which  secrete  a  homogeneous  and  elastic 

FIG.  141.  -Part  of  a  transverse  tj  j  th  j      exposed    Surface.      The   Cells 

section   of    the    heart   wall  of 

an  imperial**  larva,  cu.  cuti-  of  the  inner  layer  are  largest  in  Impenahs, 
with  a  large  central  mass  of  cytoplasm  and 
the  remainder  of  the  cell  body  so  thin  that 
it  is  sometimes  difficult  to  see  it.  The  outer 

layer  is  composed  of  smaller  and  somewhat  thicker  cells  with  smaller 
nuclei.  This  layer,  on  the  two  outer,  lower  quadrants  of  a  section  of 
the  vessel,  is  evaginated  into  a  series  of  processes  which  come  into  ex- 
tremely intimate  relations  with  a  plexus  of  tracheal  capillaries.  In 
fact,  the  finest  air  capillaries  enter  in  great  numbers  into  the  substance 
of  the  process.  The  outer  cuticle  is  not  in  full  thickness  over  these 
processes.  The  nuclei  of  the  layer  are  found  in  the  processes  also. 

The  middle  layer  of  the  vessel  consists  of  a  single  or  double  layer  of 
muscle  fibers.  They  are  clearly  striated  and  have  no  distinguishing 
cardiac  features  except,  perhaps,  a  slightly  shorter  sarcous  element.  It 
is  hard  to  distinguish  the  nuclei  of  these  fibers  from  the  nuclei  of  the 
outer  and  inner  layers  which  bear  the  cuticle. 

Amphioxus  has  a  well-defined  blood-channel  system  that  is  lined,  in 


—  mus.f. 


cle,  outer  and  inner  ;  p.,  pro- 
cess on  outer  side.  mus.  /., 
muscle  fiber.  X  900. 


CIRCULATORY  CHANNELS 


159 


end.  P. 


II.  v: 


FIG.  142.  —  Developing  blood  vessels  in  the  embryonic 
connective  tissue  of  a  rabbit,  bl.v.,  blood  vessels  con- 
taining young  blood  cells  ;  end.p.,  endothelial  pro- 
cesses. (From  "SxOHR's  Text-book  of  Histology"  by 
LEWIS.) 


all  parts,  with  a  simple  endothelium  resting  on  the  surrounding  connective 
tissues.  In  section,  these  cells  appear  as  a  line  marked  at  intervals 
by  the  thin  sections  of  their  disk-shaped  nuclei.  Only  on  the  large 
arterial  trunk  are  some  of  the  immediately  surrounding  connective- 
tissue  cells  developed  into  a  single  layer  of  muscle  fibers  placed  in  a 
circular  position. 

The  tunicates  show  also 
a  weakly  developed  blood 
vascular  system.  It  re- 
sembles that  of  Amphioxus 
closely,  and  in  both  cases  we 
must  remember  that  if  the 
animals  were  larger  and  the 
blood  vessels  consequently 
stronger,  they  would  show 
a  more  definite  and  charac- 
teristic structure. 

Turning  to  the  vertebrate 
animals,  we  find  the  best 
developed  and  best  known 

forms  of  blood-channel  structure.  We  shall  study  the  walls  of  the 
vessels  in  man  with  some  reference  to  the  Amphibian  forms. 

The  mammal  blood-vascular  system  begins  as 
a  system  of  solid  connective-tissue  cords  which, 
almost  immediately  that  they  are  formed,  become 
hollow  (Fig.  142).  This  leaves  some  question  as  to 
whether  they  were  intercellular  or  intracellular  in 
origin.  Their  subsequent  development  into  a  tube 
composed  of  many  cells  united  into  a  cylindrical 
cover  makes  it  appear  that  they  were  intercellular 
spaces  from  the  beginning. 

Figure  143  shows  a  transection  of  a  capillary 
in  an  Amphibian.  It  serves  well  to  demonstrate  a 
case  of  a  capillary  wall  whose  circumference  is  com- 
posed of  but  one  endothelial  cell.  All  larger 
vessels  show  more  than  one  such  cell  in  section. 

This  covering  of  endothelium  (Fig.  144)  is  all 
that  a  blood  vessel  actually  would  need  to  retain 
the  blood  if  the  pressure  were  always  low.  But  owing  to  the  weight 
of  the  blood  and  the  great  pressures  that  it  is  put  to  in  driving  it  on  its 
course,  the  wall  of  the  tube  is  strengthened  by  various  connective-tissue 
elements  developed  in  the  neighboring  cells.  Also  the  flow  of  blood 
has  to  be  diminished  at  times  in  most  localities,  and  this  is  done  by 


r.  N.  c. 

FIG.  143.  —  Transection 
of  a  capillary  from  a 
tadpole's  tail,  w.bl.c. 
and  r.bl.c.,  a  white  and 
a  red  blood  cell  sur- 
rounded by  the  thin 
wall  which  here  con- 
sists of  a  single  endo- 
thelial cell,  showing  its 
nucleus. 


i6o 


HISTOLOGY 


means  of  smooth  muscle  fibers  which  embrace  the  vessels.  Longitudi- 
nal fibers  appear  outside  the  circular  layer.  These  muscle  fibers  are 
the  earliest  additions  to  the  vessel  as  it  increases  in  size,  and  some 
of  the  first  of  them,  both  circular  and  longitudinal,  are  indicated  in 
Figure  144. 

This  muscle  tissue  is  added  to  by  elastic  connective  tissue  and  white 
connective  tissue  in  all  larger  vessels.     In  a  well-developed  artery  these 

tissues  are  found 
in  three  heavy 
coats,  each  of 
which  is  sub- 
divided (Fig.  145). 
The  inner  is  known 


end. 


FIG.  144.  —  Slightly  oblique  section  of  a  very  small  artery  in  the 
marrow  of  a  Guinea  pig.  To  the  right,  the  endothelial  lining 
(end.)  is  most  prominent,  with  the  circular  muscle  nuclei  (dr.  mus.) 
cut  in  transection.  The  cell  outlines  of  the  endothelial  layer  were 
added  from  another  preparation.  A  few  inner  longitudinal  muscle 
cells  were  present.  X  700. 


as  the  intima,  and 
consists  from 
within  outward  of 
the  endothelial 
lining,  a  thin,  flat 
layer  of  white  con- 
nective-tissue elements  containing  a  few  fine  elastic  fibrils,  and  a 
smooth,  even,  and  tough  elastic  membrane.  The  connective-tissue 
layer  is  drawn  out  longitudinally  into  a  thick  reticulum,  and  the  elastic 
membrane,  which  is  designated  as  the  inner  elastic  membrane,  is  usually 
thrown  into  longitudinal  folds. 

Outside  of  this  intima  comes  the  thickest  layer,  the  media.  This  is 
made  up  of  circular,  smooth  muscle  fibers  and  elastic  fibers  in  a  varying 
proportion,  sometimes  one  and  sometimes  the  other  forming  the  greater 
part  of  this  layer.  A  few  longitudinal  fibers  sometimes  occur. 

The  outermost  layer  is  the  adventitia,  which  begins  as  an  outer  elastic 
membrane  which  much  resembles  the  inner  elastic  membrane.  When 
longitudinal,  smooth  muscle  fibers  occur  in  an  artery,  they  are  placed 
just  outside  of  this  line.  Outside  of  this  we  find  a  thick  mass  of  white 
connective  tissue  that  contains  some  fine  elastic  fibers.  This  layer  acts 
more  as  a  means  of  attaching  the  vessel  to  the  surrounding  tissues  and 
carrying  its  blood  and  nerve  supply  than  as  a  wall. 

A  large  number  of  variations  may  be  seen  in  the  other  blood  vessels. 
Sometimes  one  layer  is  enormously  developed  or  almost  missing.  Veins 
as  a  rule  are  deficient  in  muscle,  though  some  veins,  on  account  of  their 
use,  resemble  arteries  in  their  structure. 

Most  vertebrates  have  two  separate  and  distinct  sets  of  blood  chan- 
nels,—  the  one  that  we  have  been  studying  and  a  group  of  somewhat 
smaller  vessels  known  as  the  lymphatic  system.  This  latter  has  a 
separate  and  subsequent  origin  as  a  series  of  intercellular  clefts  which 


CIRCULATORY  CHANNELS 


161 


appear  according  to  McClure  in  close  proximity  to  some  of  the  early 
blood  vessels,  which  latter  they  subsequently  crowd  out  of  existence  as 
active  blood  channels.  According  to  Miss  Sabin,  lymph  channels  are 
developed  as  outgrowths  of  veins  in  the  throat  and  in  the  inguinal 
region. 

Like  the  blood  channels,  the  lymph  channels  are  lined  with  a  flat 
endothelium.  Unlike  the  blood  vessels,  they  do  not  acquire  the  thick 
muscular  and  elastic  walls, 
because  there  is  no  great 
pumping  pressure  on  them 
and  the  weight  of  the  lymph 
is  not  so  great.  Where  any 
approach  to  size  is  found,  the 
strengthening  of  the  wall  is 
arranged  as  it  is  in  a  smaller 
vein. 

The  lymph  channels  com- 
municate with  the  blood  chan- 
nels directly  at  a  few  points 
where  the  lymph,  bearing  food 


—  int. 


—med. 


materials,  pours  into  the 
blood.  It  also  is  connected 
with  the  blood  space  by  tem- 
porary clefts  through  which 
plasma  and  lymph  cells  can 
pass,  but  not  the  red  blood 
corpuscle. 

The  lymph  vessels  possess 

Valves       Which       are        USUally    FlG.  I45._Portion   of   a   transaction   of  an   artery 

double    and    are    evaginated 
folds  of  the  walls.    They  also 
have  regions  where  the   pres- 
sure of  muscles  on  enlarged  parts  of  the  channels  causes,  with  the  aid 
of  the  valves,  a  slow  and  irregular  circulation. 

Technic.  — The  technic  is  usually  the  simplest.  Flemming's  fluid 
and  paraffin  sections  are  the  best  to  use  for  the  general  kinds  of  blood 
vessels.  Silver  nitrate  may  often  be  used  for  the  demonstration  of  the 
epithelial  layers  on  the  outer  and  inner  surfaces. 


45- 

from  man.  int.,  intima  ;  med.,  media  ;  ad.,  ad- 
ventitia.  (From  "SxOHR's  Text-book  of  Histology  " 
by  LEWIS.) 


LITERATURE 

Read  concerning  the  mammalian  blood  vessels  in  the  medical  histologies. 
ARGAND.    "Sur  la  Structure  des  Arteries  chez  les  Oiseaux,"  C.  R.  Ass.  Anal.  Sess.,  Vol.  VI, 
p.  90,  I9°4- 
M 


162  HISTOLOGY 

FRANZ.     "  Uber  die  Struktur  des  Herzen  und  die  Entstehung  von  Blutzellen  bei  Spin- 

nen,"  Zool.  Anz.,  Band  XXVI. 
BERGH,  R.  S.     "Uber  den  bauder  Gefasse  bei  den  Anneliden,"  Anat.  Hefte,  Band  XIV 

und  Band  XV. 

THE   CIRCULATING  MEDIA:   BLOOD 

Being  mostly  a  fluid,  the  study  of  the  blood  from  a  physiological  and 
a  chemical  point  of  a  view  would  be  more  enlightening  than  to  find  out 
what  can  be  known  of  it  from  its  histological  structure  under  the  micro- 
scope. Physiological  study  of  this  fluid,  assisted  by  chemical  studies 
and  certain  structural  studies,  have  taught  us  that  some  of  its  functions 
are,  roughly  speaking,  the  following:  — 

The  primary  transportation  of  nutriment  from  the  digestive  tract 
and  its  distribution  in  the  tissues;  the  secondary  transportation  of  food 
materials  to  and  from  storage  in  the  liver  and  elsewhere ;  the  transporta- 
tion of  oxygen  from  the  organs  of  respiration  to  the  tissues;  and  lastly, 
as  a  result  of  the  use  of  these  materials,  the  carrying  of  the  products  of 
combustion,  carbon  dioxide,  and  urea,  to  the  respiration  organs  and  the 
nephridia  respectively,  to  be  cast  out  as  waste  matter.  More  mechanical 
in  its  operation  is  the  function  of  providing  transportation,  by  floating, 
for  many  moving  cells,  the  blood  corpuscles,  that  have  duties  to  perform 
in  other  parts  of  the  body,  and  lastly,  the  blood  has  the  power  of  coagu- 
lating into  a  more  or  less  hard  mass,  for  the  purpose  of  preventing  hem- 
orrhages when  the  circulatory  system  is  cut  or  injured  at  any  point. 

In  its  origin  the  blood  must  be  looked  upon  as  derived  from  the 
mesenchymal  tissue,  part  of  whose  cells  formed  its  walls  and  others  the 
blood  cells  or  corpuscles.  By  some  investigators  it  is  thought  that  the 
plasma  as  well  as  the  blood  cells  are  derived  from  the  excavated  interiors 
of  chains  of  these  cells,  called  the  vaso-formalive  cells.  It  is  possible  that 
the  blood  elements  are  derived  from  certain  of  the  mesenchyme  cells 
that  lay  between  the  others,  which  became  the  walls  of  the  blood  channels. 

The  blood  of  different  animals  varies  much  as  to  which  of  the  various 
functions  it  performs  and  how  it  performs  them.  We  shall  take  up  some 
of  these  functions  and  discuss  the  method  of  executing  them  in  the  vari- 
ous ways  in  different  bloods,  particularly  as  to  the  structural  features 
employed. 

The  carrying  of  food  materials  (other  than  oxygen)  is  a  principal 
function  of  the  blood.  It  can  only  do  this  when  the  foods  have  been 
properly  prepared  by  the  digestive  processes.  Otherwise  the  food  mat- 
ter might  seriously  interfere  with  the  performance  of  other  duties.  The 
degree  of  this  preparation  varies  in  different  animals.  Most  foods  are 
carried  as  a  solution.  Some  are  transported  as  an  emulsion,  and  some- 
times the  blood  cells  carry  solids,  taking  them  into  their  cytoplasm  and 


CIRCULATORY  MEDIA  163 

moving  along  with  the  current.  The  blood  platelets,  which  are  small 
portions  of  protoplasm  derived  from  the  cytoplasm  of  other  cells,  may 
represent  food  carriers  in  the  mammal  blood.  Some  lower  animals 
probably  carry  most  of  their  food  in  the  blood  as  a  solution.  The  food 
material  may  bear  some  physiological  relation  to  the  blood,  which  must 
maintain  certain  conditions  in  order  to  properly  function. 

The  blood  usually  carries  the  oxygen  supply  from  the  respiratory 
surfaces  to  the  tissues  which  use  it.  The  oxygen  is  obtained  by  the  blood 
at  these  places  by  the  chemical  affinity  of  certain  of  its  constituent  sub- 
stances for  oxygen  when  they  lack  a  certain  proportion  of  it  and  are  in 
the  presence  of  any  substance  that  contains  more  than  a  certain  propor- 
tion of  it  in  the  free  state.  These  substances  also  give  it  up  to  the  tissues 
with  which  they  come  in  contact  and  which  need  it. 

One  of  the  most  prominent  of  these  oxygen-bearing  materials  in  the 
blood  is  hemoglobin,  which  exists  in  the  red  corpuscles  of  the  vertebrates 
and  free  in  the  blood  plasma  of  some  invertebrates.  Some  of  the 
invertebrates  have  other  substances  with  the  same  unstable  affinities 
for  oxygen  that  haemoglobin  has.  There  are  several  such  substances. 
They  have  other  colors  and  are  probably  not  as  efficient  as  haemaglobin. 
That  of  some  mollusks  is  called  h&mocyanine.  Sometimes  the  blood 
does  not  have  the  carrying  of  oxygen  to  do,  the  fresh  air  being  conducted 
all  over  the  body  by  fine  air  tubes  that  bring  it  directly  into  contact  with 
the  tissues.  The  insects  show  such  a  condition. 

The  carrying  of  carbon  dioxide,  as  a  waste  material  from  the  tissues  to 
the  respiratory  surfaces  for  discharge,  is  also  a  duty  of  the  blood  in  all 
forms  of  animals  except  the  insects,  where  it  is  discharged  directly  into 
the  air  tubes.  It  is  carried  in  the  blood  as  a  gas  in  solution. 

The  uric  acid  and  its  compounds  are  probably  always  carried  in  solu- 
tion by  the  blood,  which  always  has  a  certain  percentage  of  them  in  its 
body.  The  blood  is  being  constantly  relieved  of  some  of  this  burden 
when  it  passes,  in  parts  of  its  course,  regions  where  the  outer  surface  of 
the  vessels  is  lined  by  an  epithelium  that  can  extract  this  harmful  mat- 
ter from  the  blood  and  pass  it  into  spaces  that  communicate  with  the 
exterior  for  its  discharge. 

The  coagulation  of  the  blood  fluid  for  the  purpose  of  closing  any 
accidental  break  in  the  vessels  is  brought  about  by  the  formation  of 
threads  of  fibrin  in  the  blood  of  vertebrates  or  of  colloid  masses  in  the 
blood  of  some  invertebrates,  or  even  by  the  gathering  together  of  amoeboid 
corpuscles  which  intertwine  their  processes  together  to  form  an  obstruc- 
tion to  the  escaping  blood  in  still  other  of  the  lower  animals.  These 
conditions  occur  automatically  in  cases  where  the  animal  is  in  health, 
and  the  direct  stimulation  which  brings  clotting  to  pass,  seems  to  be  the 
exposure  to  the  air  and  the  arresting  of  the  blood  stream,  because  this 


i64 


HISTOLOGY 


must  occur  in  a  degree  before  the  clot  can  form.  The  process  is  not 
well  developed  in  many  animals,  to  which  a  cut  or  other  injury  becomes 
a  serious  matter.  In  the  Crustacea  there  is  a  special  arrangement  con- 
nected with  most  of  the  limbs  that  greatly  aids  the  clotting.  This  con- 
sists of  an  invaginated  ring  of  the  integument,  leaving  only  a  small  open- 
ing through  which  the  artery  and  nerve  pass.  When  the  limb  is  injured, 

and  the  animal  is  threatened 
with  a  severe  bleeding,  it  has 
the  power  to  break  the  limb 
off  at  this  point  so  that  there 
is  only  one  small  artery  and 
a  vein  to  close,  instead  of  a 
loose  system  of  lacunae.  This 
apparatus  also  serves  an- 
other purpose  with  which  we 
shall  not  here  deal. 

The  process  of  clotting 
often  results  in  a  change  of 
color  in  the  clotted  blood. 
This  color  is  very  varied  in 
the  lower  animals.  It  is 
black  in  most  of  the  insects 
and  many  shades  of  blue  or 
red,  especially  the  dull 
shades  of  those  colors,  in  the 
other  invertebrates. 

The  blood  corpuscle,  as 
has  been  said,  is  a  cell  that 

is  free  in  the  blood  and  has  assumed  a  variety  of  forms  and  structures 
according  to  the  purposes  that  it  has  to  serve  in  the  different  cases.  In 
its  simplest  form,  perhaps,  it  is  a  fairly  large  amoeboid  cell  with  a 
sharp  outline  and  a  good-sized  nucleus  that  may  be  of  very  irregular 
shape  or  even  multiple.  The  cytoplasm  is  abundant  and  filled  in 
most  cases  with  round  granules  of  large  size.  The  number  and  size  of 
the  granules  varies  in  the  same  kind  of  cell  and  is  probably  dependent 
upon  the  state  of  activity  of  the  cell  or  upon  its  age.  Notice  the  dif- 
ferent conditions  of  cytoplasmic  granulation  in  the  lobster's  blood  cor- 
puscles in  Figure  146;  also  see  the  same  feature  in  the  white  blood 
corpuscles  of  many  other  animals  (see  Figs.  139,  143).  Figure  147 
shows  one  of  the  white  blood  corpuscles  of  the  salamander.  This 
shows  an  excessive  development  of  these  structures,  and  the  same 
can  be  seen  in  many  other  forms.  This  specimen  also  shows  another 
organ  characteristic  of  the  white  corpuscle.  This  is  the  centrosome 


FIG.  146.  —  Part  of  a  blood  channel  of  the  lobster, 
containing  a  very  coarsely  granular  coagulum  of 
lymph  body,  and  blood  cells  (bl.c.)  of  several  degrees 
of  granule  development.  X  400. 


CIRCULATORY  MEDIA  1 6$ 

and  its  centriole.  Probably  no  other  somatic  cell  that  is  not  at  or  near 
a  state  of  mitotic  division  shows  this  structure  so  well.  Its  sphere  is 
free  of  the  granules  and  its  centriole  is  plainly  seen,  but  this  speci- 
men does  not  show  the  form  ascribed  to  it  by  Haidenhain,  who  de- 
scribes the  central  body  as  a  double  or  multiple  object.  The  special 
forms  of  these  cells  found  in  the  blood-forming  glands  will  be  de- 
scribed in  that  part. 

The  vertebrates  have  a  blood  cor- 
puscle that  is  used  solely  for  the  carry- 
ing of  oxygen,  and  whose  cytoplasm,  in 
consequence,  is  free  of  any  granules  and 
is  saturated  with  haemoglobin.  In  the 
mammals  this  cell  is  so  specialized  that 
it  has  lost  its  nucleus  and  the  whole 
content  is  the  oxygen-carrying  medium 
lying  in  the  plasma.  These  cells  can  be 
seen  in  the  various  tissues  in  a  number  FIG 

Of  places,  Figures  72  and  362  Showing  the  liver  of  a  salamander,  Cryptobran- 
good  examples  of  them.  cfMJ-  Centrosome  in  middle.  Nu- 

cleus to  right. 

In  the  lower  vertebrates  these  oxy- 
gen-carrying blood  cells  have  retained   the  nucleus   which,   however, 
has  lost  much  of  its  usual  structure  and  appears  contracted  and  irregu- 
lar, as  is  also  the  nucleus  about  to  be  lost  by  a  young,  red,  blood  cell  in 
a  mammal. 

All  these  red  blood  corpuscles  show  a  peculiar  glassy  and  homo- 
geneous appearance  of  the  cytoplasm  due  to  the  contained  haemoglobin. 
This  results  in  a  light  greenish  tinge  during  life,  which  appears,  however, 
to  be  red  when  seen  in  some  quantity  in  ordinary  light.  Such  cells  stain 
differently  from  other  cells  unless  the  haemoglobin  has  been  entirely 
removed,  as  happens  in  some  preparations  (Perenyi's  fluid).  Iron 
haemotoxylin  stains  them  a  jet  black,  fading  under  extreme  decolorization 
to  some  dense  shade  of  gray  or  green.  Aniline  dyes  stain  them  brilliantly. 

The  red  blood  corpuscles  of  the  salamander,  Diemyclylus,  can  be  seen 
in  stained  portions  of  the  lung  wall,  as  they  develop  from  a  somewhat 
advanced  stage  to  the  fully  matured  corpuscles  with  their  contained 
haemoglobin.  Figure  148  shows  several  of  these  stages.  It  also  shows 
that  even  after  the  acquisition  of  the  most  of  its  haemoglobin,  the  cor- 
puscle may  continue  to  divide  by  mitotic  division.  From  left  to  right 
in  the  figure  are  seen  successively  older  stages,  all  of  which  are  found 
free  in  the  capillaries.  The  earlier  history  is  not  well  known  in  this 
salamander. 

Technic.  — The  preparation  of  the  blood  for  microscopic  study  re- 
quires a  vast,  complicated,  and  delicate  series  of  processes.    There  are 


166 


HISTOLOGY 


so  many  that  one  is  at  a  loss  where  to  begin.  Blood  should  be  studied 
both  by  itself  and  in  situ  in  the  tissues.  For  the  latter,  the  ordinary 
methods  are  sufficient,  excepting  that  the  special  blood  stains,  to  be 
mentioned  later,  should  be  used  here  too.  For  study  outside  of  the  tissue, 


FIG.  148.  —  Five  stages  in  the  free  life  of  red  blood  cells  of  a  salamander,  Diemyctylus.  A,  the 
youngest  free  stage  (spindle-cell)  found  in  the  capillaries  of  the  lungs.  B  and  C,  two  suc- 
cessively older  stages  showing  the  slow  accumulation  of  htzmoglobin  in  the  peripheral  cyto- 
plasm. D,  an  older  stage  than  C,  undergoing  mitotic  division.  E,  fully  developed  and 
functional  red  blood  cell.  X  1000. 

the  blood  is  ordinarily  taken  fresh  in  small  drops  and  spread,  by  capil- 
lary attraction,  between  two  cover  glasses,  which  are  then  slid  apart 
and  the  remaining  films  dried  and  fixed  at  the  same  time  by  passing  them 
through  the  flame  of  a  spirit  lamp.  For  the  staining,  various  combina- 
tions of  some  of  the  aniline  dyes  are  used  to  get  a  differential  stain  of  the 
various  sorts  of  blood  cells  that  are  to  be  found  in  the  preparation  (see 
Lee). 

LITERATURE 

ENGEL,  S.     "  Zur  Entstehung  der  korperlichen  Elemente  des  Blutes,"  Arch.f.  mik.  Anat., 

Band  XLII,  S.  217. 
WIELOWIESJSKI,  H.  v.     "  Uber  das   Blutgewebe   der   Insecten,"   Zeit.  fur  Wiss.  Zool., 

Band  XLIII,  S.  512. 

BLOOD-FORMING  TISSUES 

The  blood  must  not  only  take  its  origin  from  the  differentiating  tis- 
sues of  the  body  and  increase  in  amount  to  suit  the  needs  of  the  growing 
embryo ;  but  provision  must  also  be  made  for  producing  further  supplies 
of  a  tissue  that  is  so  apt  to  be  lost  in  the  adult  by  accident  if  not  worn 
out  by  use.  This  idea  applies  to  the  plasma  as  well  as  the  corpuscular 
portion,  but,  owing  to  our  lack  of  knowledge  as  to  the  production  of  the 


&LOOD-FORMING   GLANDS  \6j 

plasma,  we  must  confine  our  studies  to  the  cellular  portions  in  the  few 
forms  in  which  their  origin  is  at  all  understood,  merely  saying  that  the 
plasma  is  probably  a  secretion  of  some  of  the  cells  that  line  the  channels. 
We  shall  study  some  of  the  few  known  blood  glands  and  then  speak  of 
the  first  appearance  of  blood  in  the  embryonic  tissues. 

In  general,  it  may  be  said  that  the  primitive  mode  of  producing 
new  blood  cells  is  by  a  proliferation  of  cells  from  the  inner  endothelial 
walls  of  the  blood  channels,  especially  of  the  peripheral  parts  of  the 
system.  Such  a  process  has  often  been  described  in  many  forms,  usually 
in  very  general  and  unsatisfactory  terms,  however. 

The  first  step  in  the  specialization  and  organization  of  this  function 
would  be  to  restrict  it  to  some  favorable  region  of  the  channel.  Such 
a  region  would  be  more  favorable  if  the  channel  were  modified  in  such  a 
manner  as  to  slow  the  current  and  provide  a  quiet  place  for  the  new  cor- 
puscles to  be  formed.  This  would  be  easiest  done  by  an  invagination 
of  the  endothelial  wall  of  the  channel,  and  such  simple  structures  are  to 
be  seen  in  the  crustacean  blood  glands.  We  shall  examine  one  of  these 
first. 

On  the  arteries,  especially  the  ophthalmic  artery  of  the  crayfish,  may 
be  found  a  number  of  small  irregular  glands,  each  composed  of  several 
short  acini  opening  by  a  common  and  short  duct  into  the  lumen  of  the 
artery.  The  duct  is  lined  by  the  same  intima  as  the  artery,  but  this 
structure  is  absent  at  the  entrance  to  each  acinus.  Its  place  as  a  lining 
is  taken  by  the  blood-forming  cells,  which  probably  represent  theLeidig's 


FlG.  149.  —  Section  of  a  blood  gland  from  the  ophthalmic  artery  of  a  crayfish  Astacus.  int.,  in- 
tima of  blood  vessel  extending  through  the  duct  and  partly  into  the  acinus  of  the  gland  ;  ep., 
epithelium  of  gland  one  of  whose  cells  is  beginning  a  mitotic  division  at  (mil.) ;  bl.c.,  young 
blood  cells  passing  out  of  the  gland  into  the  vessel,  v.  (After  SCHNEIDER.) 

cells  of  the  third  order  that  line  all  blood -channel  surfaces  and  secrete  the 
intima.    These  blood-forming  cells  divide  by  mitosis  and  thus  produce 


1 68  HISTOLOGY 

many  small  blood  cells  that  pass  down  the  duct  into  the  blood  stream. 
They  have  no  granules  in  their  cytoplasm  and  probably  develop  into 
both  kinds  of  corpuscles  (see  lobster's  blood)  in  the  different  parts  of 
the  channel's  system.  Small  striated  muscle  fibers  are  sometimes  seen 
in  the  walls  of  the  duct  (Fig.  149). 

A  group  of  several  other  glands  of  somewhat  doubtful  meaning  have 
been  described  as  blood-forming  glands  in  the  mollusks.  They  are 
found  in  the  neighborhood  of  the  heart  and  are  of  various  degrees  of 
concentration.  One  is  rather  widely  distributed  through  the  upper 
mantle  tissue  around  the  heart  region  of  Unio  and  other  lamellibranch 
mollusks.  Another  is  found  in  a  more  compact  form  at  the  base  of  the 
gill  in  Loligo  and  Octopus.  The  weight  of  somewhat  unsatisfactory 
evidence  has  tended  to  show  that  these  organs  are  excretory,  while  by 
some  they  are  thought  to  be  blood -making  in  function.  We  have  treated 
them  under  the  heading  of  excretion. 

An  enormous  gap  exists  between  the  one,  simply  organized  kind  of 
blood  gland  found  in  the  crayfish  or  the  Echinoderms,  and  the  great 
variety,  number,  and  complexity  of  blood  glands  found  in  the  mammals. 
In  man  these  glands  are  used  to  destroy  blood  as  well  as  to  form  several 
kinds  of  blood  cells  and  perhaps  to  perform  other  functions  as  well. 

The  fundamental  idea,  that  these  glands  are  highly  differentiated 
regions  of  the  blood-channel  wall,  is  difficult  to  maintain  and,  most  prob- 
ably, must  have  added  to  it  a  conception  of  a  part  of  these  organs  as  differ- 
entiated areas  of  mesenchymal  tissues  which,  while  in  close  functional 
relation  to  the  blood  vessels,  are  not  morphologically  a  part  of  their 
walls.  The  exact  study  of  the  blood  and  the  blood  glands  of  mam- 
mals is,  perhaps,  the  most  difficult  in  histology  as  well  as  the  one 
which  will  give  the  richest  results,  if  such  comparisons  may  be  per- 
mitted. Its  chief  difficulty  and  interest  lie  in  the  fact  that  its  cellular 
elements  are  movable  during  the  same  time  that  they  are  changing,  which 
makes  their  history  very  hard  to  put  together  by  studying  dead  sections. 
The  structures  cannot  be  studied  in  situ  during  life. 

We  shall  study  a  smaller  lymph  node,  or  nodule  as  an  example  of 
one  of  the  more  primitive  blood  glands  in  man.  Such  a  blood  center 
may  be  found  in  many  positions  in  the  body  and  appears  macroscopically 
as  a  small  lump  of  tissue  with  several  blood  and  lymph  vessels  entering 
or  leaving  it. 

A  gland  of  this  kind  begins  as  a  differentiation  of  the  mesenchyme 
in  the  neighborhood  of  some  blood  and  lymph  vessels.  Branches  of 
both  kinds  of  vessels  enter  the  mass.  The  blood  vessels  enter  as  arteries 
and,  after  forming  a  capillary  circulation  in  the  pulp,  return  as  veins 
from  the  same  point.  This  point  is  called  the  hilum.  The  lymph  chan- 
nels also  enter  into  the  pulp  and  form  a  wide-meshed  plexus  in  its  sub- 


BLOOD-FOAMING    GLANDS  169 

stance  as  well  as  a  sinus-like  space  that  extends  all  over  the  periphery. 
The  afferent  lymph  vessels  enter  at  one  side  of  the  nodule  and  the  efferent 
vessels  pass  out  at  the  other  (Fig.  150). 

A  capsule  of  white  connective  tissue  is  formed  around  the  whole 
mass,  and  when  the  nodule  attains  a  certain  size,  plate-like  trabeculae 
of  this  sheath  are  pushed  into  it  to  provide  its  soft  tissues  with  mechani- 
cal support.  Elastic  fibers  and  smooth  muscle  fibers  may  develop  in 


tr. 


Lv. 


FIG.  150.  —  Diagram  of  a  lymph  gland,  aff.l.v.,  afferent  lymph  vessels  ;  eff.l.v.,  efferent  lymph 
vessels  ;  bl.v.,  blood  vessels  ;  per.s.,  peripheral  sinus  ;  l.s.,  lymph  sinus  ;  no.,  lymph  nod- 
ule ;  med.c.,  medullary  cord  ;  tr.,  trabecula.  (From  "STOHR'S  Histology"  by  LEWIS.) 

the  connective  tissue  and  trabeculae  of  the  larger  lymph  glands.  The 
lymph  sinuses  usually  touch  the  trabeculae  and  follow  their  course. 
They  thus  come  to  lie  between  a  trabecula  and  a  mass  of  lymph  tissue. 
They  are  separated  from  the  lymph  tissue  by  a  layer  of  flat  cells  which 
may  be  considered  to  be  the  walls  of  the  lymph  sinus. 

The  excavation  of  the  lymph-cell  mass  (or  lymph  tissue)tby  the 
lymphatics  leaves  it  in  a  series  of  masses  which  when  rounded  are  known 
as  the  lymphatic  nodules,  and  when  elongate  as  the  lymphatic  cords. 
The  arteries  and  veins  are  found  inside  these  masses.  The  lymph  enters 
such  a  gland  and,  in  flowing  through  its  sinuses,  has  added  to  its  current 
numerous  lymph  cells  which  creep  out  of  the  lymph  mass,  in  which  they 
had  their  origin  by  cell  division  and  development,  and  pass  out  of  the 
gland  with  the  lymph. 

Bacteria  and  other  harmful  foreign  bodies  may  be  ingested  by  lymph 
cells  directly  in  the  lymph  glands.  This  process  may  go  on  until  the  usual 


1 70 


HISTOLOGY 


structural  relations  are  so  disturbed  and  the  channels  become  so  ob- 
structed that  the  gland  breaks  down. 

Where  many  lymph  nodules  are  gathered  together  into  a  single  mass 
they  acquire  a  cord-like  reticulum  of  lymphoid  tissue  in  addition  to  the 
lymph  nodules  as  seen  in  our  first  example.  In  such  a  lymph  node 

the  nodules  are  placed  near  the 
periphery  and  the  cord  mass  oc- 
cupies the  center  (Fig.  151). 

The  function  of  disposing  of 
harmful  matter  extends,  in  the 
lymph  glands,  to  the  destruction 
of  broken-down  red  blood  cor- 
puscles, as  well  as  the  formation 
of  lymphocytes.  This  function 
predominates  in  some  blood 
glands,  which  resemble  lymph 
glands  except  that  where  lymph 
alone  flowed  into  the  peripheral 
sinuses  of  the  real  lymph  gland, 
red  blood  flows  into  the  sinuses 
of  this  kind,  and  they  are  called 
hfBmal  glands.  Their  function 
may  be  spoken  of  as  blood 
filtering. 

The  largest  gland  of  a  lym- 
phatic nature  in  the  vertebrate 
animals  is  the  spleen.  This 

organ  may  be  looked  upon  as  a  collection  of  many  somewhat  special- 
ized lymph  nodules  lying  in  a  much  larger  mass  of  blood -removing 
tissue  called  the  splenic  pulp  or  medulla.  This  splenic  pulp  must  be 
compared  with  the  peripheral  sinus  found  in  the  lymphatic  nodule,  or 
more  exactly  and  closely  with  the  similar  sinus  found  in  the  haemal 
gland. 

The  splenic  pulp  is  arranged  in  a  number  of  radial  masses,  each  con- 
taining several  lymphatic  nodules  (here  called  Malpighian  bodies)  and 
each  supplied  by  an  arterial  branch.  A  vein  also  collects  many  small 
branches  which  originate  in  the  pulp  and  carries  the  blood  out  near  the 
point  (the  hilum)  at  which  the  artery  entered  (Fig.  152).  On  entering 
the  spleen  pulp,  each  artery  is  closely  invested  with  a  layer  of  lymphatic 
tissue  which,  in  man,  is  very  thin  and  is  expanded,  at  certain  points  only 
on  the  branches,  to  form  the  lymph  nodules.  The  artery  carries  blood 
to  the  nodule,  where  some  of  it  is  diverted  into  capillaries  in  the  lymphatic 
tissue,  while  the  rest  is  carried  distally  by  the  arterial  branches  to  the 


FIG.  151. — A  section  of  lymphatic  tissue  as  it 
ordinarily  appears.  Two  nodules  shown. 
Lymph  cords  massed  in  homogeneous  ap- 
pearing tissue.  From  one  of  the  larger 
masses  of  lymph  tissue  near  the  appendix  of 
the  cat.  X  100. 


BLOOD-FORMING    GLANDS 


171 


pulp.    The  pulp  is  divided  into  rather  indistinct  regions,  each  of  which 
is  supplied  with  a  fine  arterial  branch. 

Both  the  nodular  capillaries  and  the  pulp  branches  discharge  the 
blood  into  the  pulp,  with  which  it  mixes  and  from  which  it  is  afterwards 
drawn  out  by  the  veins  which  originate  in  this  pulp.  The  exact  degree 
of  direct  connection  between  artery  and  vein  through  this  pulp  is  a  sub- 
ject of  much  doubt  and  of  some  controversy. 

The  arteries  show,  near  the  point  at  which  they  terminate  in  the  pulp, 
a  thickened  wall  which  is  supposed  to  regulate  the  amount  of  blood  that 
is  discharged  into  the  pulp. 
The  veins  show  at  or  near 
their  point  of  origin  a  basket- 
work,  or  open  wicker-like  ar- 
rangement, of  the  circular  and 
longitudinal  fibers,  through 
which  the  blood  may  enter 
them  from  the  pulp.  These 
fibers  are  not  muscle  fibers, 
but  contractile  endothelial 
elements. 

The  pulp  itself  is  com- 
posed of  a  reticular  connec- 
tive-t  issue  framework  in 
whose  meshes  are  to  be  found 
the  pulp  cells.  The  whole  a.  1 

Mai*  c. 

FIG.    152.  —  Diagram   of  a  portion    of    spleen,    a., 
^^  with  branches  to  the   compartment  units; 

v<  vein  with  its  collecting  branches  ;  Mai.c.,  Mai- 

pighian   body  or  lymph  node  ;    sp.c.,  spleen   pulp 


-sp.  c. 


****** 


mass   is  infiltrated   with    the 

blood    Cells   which    have    been 
tVirnwn  i'nrn  it  hv  trio  artPnVc 

thrown  into  it  by  trie  arteries. 

As  may  be  Seen  in  Figure  153, 

from  a  salamander,  some  of 

these  red  corpuscles  swell  up 

and  break  down  and  are  probably  ingested  by  white  corpuscles  or 

phagocytes. 

The  structure  of  the  spleen  would  possibly  permit  of  the  fellowing 
processes  to  take  place  in  it:  First,  the  production  of  lymph  cells 
in  the  lymph  nodules  and  their  passage  into  the  pulp.  Second,  the 
passage  into  the  pulp  of  red  blood  corpuscles  many  of  which  (the 
broken  or  diseased  or  "worn-out"  ones)  are  disintegrated  and  ab- 
sorbed by  the  lymph  cells,  which  then  pass  out  through  the  terminal 
veins  together  with  those  red  corpuscles  which  have  escaped  destruc- 
tion. 

The  three  (in  theory  but  two)  kinds  of  glands  mentioned  above 
operate  to  produce  white  blood  corpuscles  (lymph  tissue]  and  to  destroy 


172 


HISTOLOGY 


bl.c. 


red  corpuscles  and  foreign  matter  (haemal  tissues  of  haemal  glands  and 
pulp  of  spleen).  We  shall  now  examine  the  third  variety  of  blood  gland, 
that  which,  among  its  other  duties,  is  responsible  for  the  production  of 
the  red  corpuscles.  This  is  the  red  marrow  of  the  bones.  It  is  devel- 
oped among  the  fat  cells  of  which  the  white  or  yellow  marrow  is  com- 
posed, and  consists  of  a  few  connective- tissue  cells  arranged  as  a 
reticulum  in  whose  meshes  the  marrow  cells  lie.  These  cells  are  de- 
scendants of  the  perichon- 
dral  cells  that  first  invaded 
the  bone  during  its  de- 
velopment or  during  its 
reconstruction  from  a  car- 
tilage. 

Besides  the  storing  of 
nutrient  materials  in  the 
form  of  fat,  the  marrow 
has  the  work  of  excavat- 
ing the  bone  and  some- 
times of  forming  new 
bone.  These  two  func- 
tions are  treated  of  else- 
where, and  we  shall  study 
the  red  marrow  here  with 
a  view  to  understanding 
how  it  is  able  to  furnish 
the  blood  with  new  red 
corpuscles. 

The  majority  of  cells 
found  in  a  section  of  marrow  are  of  medium  size  and  possess  a  large 
round  or  slightly  irregular  nucleus  with  the  form  of  chromatin  reticu- 
lum ordinarily  seen  in  young  cells  (Fig.  154).  These  are  called  the 
myelocytes,  and  they  retain  their  numbers  by  mitotic  division.  In  their 
earlier  stages  they  are  somewhat  smaller,  the  nucleus  is  proportionally 
larger,  and  the  cells  are  called  premyelocytes .  These  cells  are  probably 
the  producers  of  the  red  blood  corpuscles.  These  are  formed  by  the 
shrinking  of  the  nucleus  and  its  final  extrusion  from  the  myelocyte. 
During  this  time  the  cell  is  called  an  erythroblast,  a  normoblast,  and 
finally,  when  the  nucleus  is  dissolved  or  extruded,  an  erythrocyte  or  red 
blood  corpuscle.  The  early  origin  of  blood  in  the  embryo  is  the  same, 
except  that  it  must  come  from  the  blood  islands  which  are  circumscribed 
areas  of  mesodermal  tissue.  The  central  cells  of  this  tissue  become 
erythroblasts  and  go  through  the  same  changes  that  the  myelocytes  do. 
According  to  Bunting  "  in  case  of  extensive  injury  to  the  marrow,  the 


w.  bl  c. 

FIG.  153.  —  Small  portion  of  a  section  through  the  sple- 
nic pulp  of  a  salamander,  bl.c.,  normal  blood  cells  ; 
deg.bl.c.,  degenerating  blood  cells  ;  sw.U.c.,  much  swol- 
len blood  cell  about  to  be  destroyed  ;  pul.c.,  pulp  cells; 
•w.bl.c.,  white  blood  cell  or  phagocyte.  X  870. 


BLOOD-FORMING    GLANDS 


173 


spleen   may  take  on  the  haemopoietic  function,  the  new  cells  being 
formed  in  the  sinuses  of  the  organ." 

Technic.  —  While  the  use  of  smear  preparations  (made  as  were  the 
blood  films  in  the  last  exercise)  is  valuable  to  separate  and  study  the 
individual  cells,  it  should  be  borne 
in  mind  that  to  get  any  real  rela- 
tions of  the  various  kinds  of  cells  to 
one  another  it  is  necessary  to  use 
the  best  sections.  As  this  study  of 
the  structural  relations  is  the  only 
way  in  which  we  can  understand 
the  production  of  the  blood,  the 
section  method  should  be  used 
almost  alone.  The  smear  prepa- 
rations may  be  used  for  com- 
parison. 


ta   He. 


g.e.' 


LITERATURE 


S.  474, 


FlG.  154. — Several  cells  sketched  in  situ  in  a 
section  of  marrow  from  the  Guinea  pig's 
humerus.  g.c.,  giant  cell  ;  my.,  myelocytes  ; 
e.b.,  erythroblast  ;  w.bl.c.,  white  blood  cell  ; 
r.U.c.,  red  blood  cells  ;  mi.,  mitosis  ;  e-c., 
erythrocyte.  X  1000. 


SCHNEIDER,  K.  A.     "Histologie, 

"Blutdriise  der  Astacus." 
SAXER,  FR.     "  Uber  das  Entwicklung  und 

den  Bau  der  normalen  Lymphdriisen 

und  die  Entstehung  der  roten  und  weissen  Blutkorperchen,"    Anal.  Hefte,  No.  6, 

1896. 
WHITE,  F.  G.     "Haemolymph  Glands  in  Domestic  Animals,"  Am.Journ.  of  Anat.,  Vol. 

Ill,  p.  8. 
MALL,  F.  P.     "The  Lobule  of  the  Spleen,"  Johns  Hopkins  Hospital  Bulletin,  Vol.  IX, 


BUNTING,  C.  H.     "  Formation  of  Blood  by  the  Spleen,' 
VIII,  No.  5,  1906. 


Journ.    Exp.  Medicine,  Vol. 


CHAPTER    XIII 
NERVE   TISSUES 

THE  nerve  cells  are  the  cells  that  put  an  organism  into  communica- 
tion and  correlation  with  outer  chemical,  physical,  and  mechanical 
conditions.  In  order  to  perform  this  duty  they  must  be  able  to  do  three 
things :  — 

Firstly,  to  perceive  or  be  stimulated  by  the  outer  conditions  directly, 
or  indirectly  through  the  stimulus  of  another  nerve  cell,  cell-product, 
or  foreign  substance,  —  function  of  perception. 

Secondly,  to  transfer  the  stimulus,  so  received,  as  an  impulse  through 
the  cell  substance  to  some  other  surface  of  its  cytoplasm  which  is  in 
nervous  contact  with  the  cell  or  cells  that  are  to  be  communicated  with, 
—  function  of  conduction. 

Thirdly,  to  discharge  the  impulse  to  this  other  cell  or  cells  as  a  stim- 
ulus, —  function  of  stimulation. 

It  is  probable  that  all  cells  of  an  unspecialized  character  in  the  lower 
animals  have  more  or  less  of  these  three  powers,  and  it  is  only  when  the 
cell  is  modified  to  perform  the  function  specifically  that  we  recognize 
them  as  nerve  cells.  The  specialization  consists  of  the  acquisition  of 
three  different  kinds  of  cell-organs  by  the  cytoplasm  of  the  respective 
cells:- 

1.  Of  the  development  in  the  cytoplasm  of  a  perceptory  organ  to 
receive  the  stimulus.    This  organ  is  a  modification  of  the  cytoplasm 
at  some  favorably  situated  point  on  the  surface,  and  consists  in  different 
cells  of  a  great  variety  of  rods,  hairs,  plates,  cones,  fibrils,  protoplasmic 
processes,  etc.,  which  are  modified  to  suit  the  conditions  met  with  (see 
Fig.  155,  upper  arrows). 

2.  Of  the  development  in  the  cytoplasm  of  a  number  of  fine  fibrils, 
the  neuro-frbrils  and  other  structures  to  be  used  in  carrying  the  resulting 
impulse  to  the  other  end  or  pole  of  the  cell.    This  pole  is  in  contact  with 
some  other  cell  or  cells  with  which  it   is   intended  to   communicate. 
The  distance  traversed  causes  the  communicating  cytoplasm  to  form 
a  longer  or  shorter  fiber  (Fig.  155,  B,  C,  D,  E,  and  F). 

3.  Of  the  formation,  at  the  surface  of  this  point,  of  an  end-organ  or 
end-plate  that  is  used  to  discharge  the  impulse  as  a  stimulus  to  the  other 

174 


NERVE    TISSUES 


175 


cell,  which  may  be  a  nerve  cell,  a  muscle  cell,  or  a  cell  of  some  other  kind 
(Fig.  155,  B,  etc.).  That  the  perceptory  end-organs  and  the  discharging 
end-organs  are  specific  and  necessary  structures  is  proved  by  the  fact 
that  the  cell  cannot  operate  without  them  or  when  they  are  injured 
or  diseased.  One  or  both  of  them  can  be  regenerated  in  some  ani- 
mals. Such  a  cell  with  its  processes  and  end-organs  is  known  as  a 
neuron. 

The  nerve  cell,  which  is  usually  large  and  well  developed,  may  have 
a  great  variety  of  forms.  It  may  be  compact  and  but  little  elongated 
(Fig.  155,  A},  as  are  many  surface  nerve  cells  used  to  perceive  mechanical 


Ilil  1    111 


FIG.  155.  —  Diagrams  of  different  kinds  of  nerve  cells.  External  arrows  point  to  receiving  or 
perceptory  surface;  internal  arrows  show  discharging  or  stimulatory  surfaces  of  cells.  A,  a 
nerve  cell  with  no  process;  B,  the  same  with  its  discharging  surface  or  organ  on  a  process; 
C,  both  end-organs  on  processes;  D,  the  same  with  impulse- path  independent  of  the  cell 
body ;  E,  multiple  perceptory  organs ;  F,  both  end-organs  multiple. 

stimuli.  As  it  is  primarily  intended,  however,  for  communication  be- 
tween more  or  less  widely  separated  parts  of  the  body,  it  is  almost  always 
extended  by  means  of  drawn-out  processes  of  the  cytoplasm  of  its  cell 
body  (Fig.  155,  B,  C,  D,  E,  F).  The  perceptory  and  stimulating  sur- 
faces of  the  cell  are,  of  course,  placed  at  the  ends  of  the  processes  in  order 
that  they  may  be  next  to  the  points  to  be  communicated  with.  There 
may  be  one  process,  placed  either  at  the  perceptory  pole  of  the  cell  (Fig. 
155,  A)  or  at  the  discharging  end  (Fig.  155,  C).  Oftener  there  are  two 
processes  of  unequal  length,  one  placed  at  each  pole.  The  poles  and 
processes  often  are  both  on  one  side  of  the  cell  (Fig.  155,  D).  The  pro- 
cesses are  sometimes  single  but  often  multiple  at  one  end  or  the  other. 


176  HISTOLOGY 

They  frequently  branch  once  or  many  times,  sometimes  forming  a  very 
dense  and  extensive  network.  One  process  or  its  multiple  parts  brings 
the  impulse  into  the  cell  and  is  called  the  afferent  process  or  dendrite. 
The  impulse  passes  through  (or  by)  the  cell  body  and  is  carried  away 
to  its  destination  by  the  other  process,  which  is  called  the  efferent  process 
or  neurite.  The  direction  of  the  impulse  in  the  nerve  cell  is  invariably 
the  same,  from  perceptory  end  to  discharging  end,  and  it  is  never  reversed. 
The  nature  of  this  impulse  is  not  known.  Its  time  reactions  and  other 
experiments  show  that  it  is  not  specifically  electrical.  It  is  not  the  stim- 
ulus itself  carried  through  the  cell,  but  a  reaction  of  the  cell  to  the  stimulus. 
The  impulse  can  be  controlled  and  elaborated  by  the  cell  and  may  be 
retarded  or  suppressed  or  repeated  in  rhythmic  order  or  even  accu- 
mulated and  augmented.  We  are  thus  unable  to  arrive  at  any  conclu- 
sion as  yet  concerning  the  exact  physical  and  chemical  conditions  that 
underlie  the  operation  of  impulse  conduction.  It  is  the  dendrite  that  is 
usually  multiple,  especially  in  the  motor  cells  and  communicatory  nerve 
cells  that  receive  the  stimulation  from  other  nerve  cells.  It  is  commonly 
single  in  the  perceptory  or  sense  nerve  cells. 

The  cell  body  is  usually  large  and  distinct.  It  is  possessed  of  all  the 
ordinary  cell-organs  necessary  for  its  trophic  maintenance  and  in  many 
cases  is  multinuclear. 

The  nerve  cells  have  been  more  changed  in  their  positions  in  the  body, 
perhaps,  than  any  other  tissue.  These  changes  can  also  be  traced  better 
and  serve  to  explain  many  features  of  form  and  function  which  would 
otherwise  remain  unsolved.  The  nerve  tissues  originated  phylogeneti- 
cally  on  the  surface  of  the  body  and  were  primarily  ectodermal  in  char- 
acter. T.his  would  be  a  logical  assumption  even  without  further  evidence, 
because  it  was  only  such  cells  as  were  on  the  outside  of  the  body  that  were 
in  contact  with  changing  conditions  which  they  must  perceive  and  to 
which  they  must  adapt  themselves.  The  primitive  nerve  cell  was  prob- 
ably a  perceptory  cell  with  weak  powers  of  conduction  and  stimulation, 
which  two  latter  powers  must  always  follow  the  first. 

From  this  superficial  position  all  nerve  cells  but  those  that  must  be 
on  the  outside  (or  near  enough  to  it  to  be  accessible  to  the  stimuli)  have 
retreated  into  the  most  inner  and  best-protected  positions  possible.  This 
is  seen  to  advantage  in  the  ontogeny  of  nearly  all  of  the  higher  forms. 
The  most  primitive  manner  of  retreat  is  for  the  cell  body  to  grow  down 
from  the  periphery  and  leave  its  perceptory  process  and  end-organ  at  the 
surface.  When  the  primitive  nerve  cells  became  differentiated  to  perform 
the  three  nerve  functions  specifically,  many  of  the  cells  moved  inside  and 
took  their  stimuli  from  the  perceptory  cells  that  were  still  situated  at  the 
surface.  The  inner  cells  also  acquired,  by  differentiation,  new  powers 
which  have  resulted  in  the  wonderful  nervous  systems  of  many  animals, 


NERVE   TISSUES  177 

especially  of  man.  These  inner  nerve  cells  form  the  central  nervous 
system,  and  those  remaining  in  direct  contact  with  the  exterior  form  the 
perceptory  nervous  system  or  sense  organs. 

Where  large  numbers  of  nerve  cells  were  to  be  retired  from  the  sur- 
face at  once,  entire  parts  of  this  surface  were  invaginated  and  the  whole 
mass  thus  carried  inside  as  in  the  vertebrate  brain  and  the  cephalopod 
brain.  These  form  communicatory  and  motor  centers.  Many  large 
sensory  areas  that  can  be  reached  inside  the  body  (through  specially 
developed  outer  tissues)  by  vibrations  of  the  ether  and  of  air  or  other 
matter  are  also  invaginated  for  the  protection  of  their  delicate  cells 
(retina,  organ  of  Corti). 

The  nerve  cells  rarely  work  alone  (motor  cells  of  some  jellyfish). 
For  the  most  part  they  are  arranged  in  chain-like  pathways  through  the 
body.  The  links  of  such  a  chain  are  the  individual  neurons  arranged 
with  the  discharging  end  of  each  one  in  close  proximity  or  contact  with 
the  perceptory  organ  of  the  next,  so  that  an  impulse,  beginning  at  the 
perceptory  end  of  the  first  cell  in  the  line  (this  must  be  a  cell  specially 
modified  to  take  a  stimulus  from  some  outer  conditions),  will  travel  the 
entire  length  of  the  chain,  ending  at  the  discharging  end  of  the  last 
one  as  a  motor  stimulus  to  a  tissue  cell.  The  impulse  may  be  divided 
and  be  discharged  from  the  end-organs  of  a  number  of  branches  at  the 
same  time. 

The  circuits  vary  from  short  ones  composed  of  two  or  three  neurons 
to  long  ones  composed  of  many.  These  paths  are  arranged,  in  some 
cases,  so  that  the  longer  ones  may  be  short-circuited.  As  some  of  the 
nerve-cells  can  modify  and  act  upon  the  impulses  that  pass  through  them, 
this  becomes  true  also  of  the  entire  circuit.  Some  of  them  are  very  com- 
plicated and  are  arranged  for  the  performance  of  the  mental  processes 
in  the  forms  that  possess  the  power  of  thinking.  The  method  of  this 
performance  is  not  understood. 

The  maintenance  of  these  closely  related  pathways  through  the  nerv- 
ous system  depends  upon  the  exact  and  accurate  working  of  its  units, 
the  nerve  cells.  These  cells,  called  the  neurons,  will  always,  in  health, 
carry  an  impulse  along  its  appointed  path  and  deliver  it  at  a  certain 
point  or  points.  It  will  not  allow  it  to  "leak"  into  the  neighboring  cells 
or  tissues  until  it  is  discharged  into  the  cell  for  which  it  is  intended,  and 
it  will  always  be  proportional,  in  kind  and  degree,  and  within  more  or 
less  narrow  limits,  to  the  stimulus  that  caused  it.  This  proportion  is  not 
always  a  direct  ratio.  Some  neurons  can  receive  a  wider  range  of  stimuli 
than  others  which  are  more  highly  specialized  and  consequently  more 
restricted  in  their  repertory.  A  stimulus  too  weak  or  too  strong  will 
produce  no  impression  whatever.  The  aggregate  of  body  surfaces  from 
which  the  nerve  cells  receive  their  perceptory  stimuli  is  known  in  neu- 


1/8  HISTOLOGY 

rology  as  the  periphery;  and  those  other  surfaces,  which  have  retired 
into  the  interior  to  form  the  ganglia  and  brains  and  receive,  elaborate, 
and  send  out  the  reports  of  the  sensory  nerve  cells  as  motor  commands, 
are  known  as  the  central  nervous  system.  This  conception  of  the  inde- 
pendent and  exact  action  and  interaction  of  the  neurons  is  known  as 
the  "neuron  theory,"  and  is  supposed  to  depend  upon  the  absolute  nerv- 
ous separateness  of  each  and  every  neuron,  no  matter  how  intimately 
they  may  be  united  physically.  In  the  light  of  recent  research  it  is  pos- 
sible that  the  unit  of  nerve  activity,  while  usually  a  neuron,  is  sometimes 
a  part  of  a  neuron  or  even  formed  by  two  or  more  of  them  acting  in 
unison. 

Most  nerve-paths  have  common  meeting  grounds  with  one  or  more 
others  for  the  exchange  of  the  nerve  impulses.  Here  are  assembled 
the  perceptory  and  discharging  organs  of  larger  or  smaller  groups  of 
neurons  together  with  the  cell  bodies  of  such  as  have  the  cell  body  near 
either  end -organ.  Some  of  the  neurons  are  confined  entirely  to  this 
region,  and  the  whole  mass  together  with  certain  connective  tissue  and 
circulatory  elements  is  known  as  a  ganglion.  Some  ganglia  are  composed 
principally  of  nerve  cells  whose  perceptory  and  discharging  end-organs 
are  one  or  both  widely  remote  from  the  region,  in  other  ganglia  or  at  the 
periphery. 

Some  ganglia  may  be  small  and  homogeneous  as  to  the  kind  of  cells 
that  are  found  in  them,  others  larger  or  containing  a  greater  variety  of 
cell  elements.  This  condition  is  true  of  the  greater  number  of  animals. 
In  some  higher  forms  numbers  of  ganglia  of  several  different  kinds  are 
collected  into  large  central  masses,  which  are  closely  assembled  in  some 
central  region  to  form  the  central  nervous  system,  as  the  brain  in  the 
mammals  and  man.  The  nerve  cells  and  their  products  greatly  outweigh 
everything  else  in  such  centers,  other  elements  being  neuroglia,  a  little 
connective  tissue,  and  a  considerable  amount  of  circulatory  medium. 
The  vertebrates  and  cephalopod  mollusks  possess  well-developed  exam- 
ples of  such  brains.  Some  of  the  Arthropoda  are  only  a  step  behind  in 
this  respect. 

A  classification  of  the  nerve  tissues  and  their  cells  according  to  their 
use  is  the  one  we  shall  make  use  of  here  as  far  as  possible.  According  to 
the  specialization  of  one  or  the  other  of  the  three  fundamental  cell-organs, 
the  cells  (and  the  tissues)  will  be  spoken  of  as  perceptory,  communicatory, 
or  motor.  Of  course  all  nerve  cells  can,  as  has  been  stated,  perceive,  and 
it  must  be  explained  that,  in  this  case,  by  perceptory  cells  are  meant  all 
neurons  that  receive  first  hand  a  perception  of  exterior  conditions  through 
a  chemical  or  mechanical  or  physical  stimulus.  (The  distinction 
between  physical  and  mechanical  is  here  used  for  convenience.)  These 
three  forms  of  cells  will  be  considered  as  the  tactile  (including  the  static 


THE  NERVE    CELL  179 

and  auditory],  the  olfactory  (including  the  gustatory],  and  the  "visual  cells 
and  tissues. 

Included  in  the  communicatory  cells  are  those  that  receive  their  stim- 
ulus from  another  neuron  or  group  of  neurons  and  transmit  it  as  an  im- 
pulse to  still  other  neurons  in  a  chain.  Such  cells  are  able  to  perceive 
and  to  stimulate  other  nerve  cells.  They  exist  chiefly  to  act  as  transmit- 
ting units  between  other  neurons.  As  has  been  said,  some  of  them  can 
manipulate  the  impulse  in  transit,  a  subject  we  know  but  little  about.  In 
cases  where  the  nerve-chain  is  composed  of  only  one  neuron,  no  such 
specialization  has  taken  place,  and  the  one  cell  performs  all  three  func- 
tions. Two  neurons  in  a  chain  mean  that  the  function  of  communica- 
tion is  unspecialized.  The  motor  neurons  stimulate  the  muscle  cells 
and  other  cells  into  action.  This  is  their  chief  duty  notwithstanding 
that  they  must  also  perceive  and  conduct.  In  all  but  the  rarest  cases 
the  cell  body  lies  in  a  ganglion  or  central  ganglion  as  the  anterior  horn 
of  the  cord  in  the  vertebrates.  One  exception  is  formed  in  some  medusae 
in  which  the  cell  body  of  a  neuron  with  its  perceptory  organ  lies  in  the 
periphery  and  receives  stimuli  which  pass  direct  to  the  muscle.  Other- 
wise the  cell  usually  has  a  number  of  processes  that  receive  the  impulse. 
These  are  the  dendrites.  They  may  be  large  and  branched  as  in  the 
electric  motor  cells  of  fishes  which  furnish  the  stimulus  to  the  electro- 
plaxes  found  in  these  forms.  Also  in  the  cerebellum  of  man  when  the 
Purkinje  cells  have  even  more  branched  processes. 

Technic.  —  Very  little  can  be  learned  about  the  real  structure  of  nerve 
tissues  from  the  study  of  ordinary  sections  stained  in  haematoxylin  and 
other  ordinary  stains.  To  get  any  idea  of  the  real  disposition  of  the  ele- 
ments one  must  resort  to  a  very  large  number  of  special  and  difficult 
processes  that  require  time  and  experience  for  their  proper  performance. 
Some  of  these  methods  will  be  mentioned  under  the  several  parts  of  the 
chapter,  but,  for  the  most,  the  reader  is  referred  to  LEE. 

LITERATURE 

JOHNSON,  J.  B.     "Text-book  of  Neurology,"  1906.     Saunders  &  Co.,  Philadelphia. 
SCHNEIDER,   K.  C.     Several  sections  of  the  "Lehrbuch  der   vergl.  Histologie.1"     Jena, 

G.  Fischer,  1902. 
BARKER,  L.     "The  Nervous  System."     New  York,  1899.     D.  Appleton  &  Co. 


THE  NERVE   CELL 

Although  the  neuron  is  probably  as  highly  specialized  a  cell  as  there 
is  in  the  body,  still  it  must  execute  the  ordinary  processes  of  assimilation, 
of  respiration,  excretion,  etc.,  which  all  cells  are  constantly  performing. 
It  has  the  more  of  these  to  do,  perhaps,  on  account  of  its  large  size  and 


i8o 


HISTOLOGY 


long  processes,  as  well  as  on  account  of  its  many  and  intense  activities. 

It  is  not,  therefore,  all  drawn  out  into  processes,  even  in  its  most  differ- 
entiated forms,  but  retains  a 
large  portion  of  its  cytoplasm 
for  the  undifferentiated  trophic 
functions.  This  part  of  the  cy- 
toplasm, the  cell  body,  possesses 
in  its  mass  and  usually  near  the 
center  a  large,  well-formed  nu- 
cleus (see  Fig.  156). 

The  nucleus  in  general  re- 
sembles that  of  an  ovum  or 
spermatogonium.  It  is  spheri- 
cal (Figs.  156,  157,  and  158)  or 
irregularly  rounded  (Figs.  160 
and  161).  The  nuclear  mem- 


lymph   and   blood   channels;   conn.t.fi.,  connec-    fined,   and  while  a  distinct 
tive-tissue  fibrils  entering  to  support  the  cell  body.    network  js  not  always  yisible>  it 

is  typical  in  most  nerve  cells,  especially  where  the  chromatin  is  distrib- 
uted in  masses  of  any  considerable  size  (see  Fig.  158). 

The  chromatin  is  arranged  in  many  ways.     It  may  be  distributed 
throughout  the  nucleus  in  such  fine  particles  as  to  appear  merely  as  a 


ch  nu, 


FIG.  157.  —  Large  motor  ganglion  cell  from  the  electric  lobe  of  the  brain  of  the  torpedo,  Tetron- 
arce.  imp.c.,  implantation  cone;  ch.nu.,  chromatin  knot;  den.,  dendrites  shown  at  their 
beginning.  The  chromatin  nucleolus  is  always  placed  opposite  to  the  nucleolus  or  plasmo- 
some  and  the  axis  thus  formed  is  the  same  in  all  the  electric  cells. 

ground  color  when  stained.  More  often  it  appears  as  a  considerable 
number  of  fine  but  visible  particles  of  varying  sizes  strung  out  on  the 
linin  network  and  especially  at  the  intersections  of  the  fibrils  (see  Fig. 


THE  NERVE    CELL 


181 


158).  Another  common  type  of  chromatin  distribution  is  to  be  seen 
in  the  ganglion  cells  from  the  stellate  ganglion  of  the  squid  and  the 
electric  lobes  of  the  tor- 
pedo's brain  where  some, 
comparatively  few,  of  the 
chromatin  masses  are 
found  to  be  very  much 
larger  than  the  others  and 
to  form  centers  around 
which  these  smaller,  dust- 
like  particles  have  gath- 
ered in  clouds  that  thin 
out  toward  the  edge  (see 
Figs.  156  and  157).  In 

the  last  example,  the    tor-      FIG.  158.— Nerve  cells  from  a  sympathetic  ganglion  (cer- 

pedo's     electric     Cell,     Still          vical>  of  the  muskrat,  Fiber,     tr.,  trophospongia,  nutri- 

,  ,         ,  .  ent  channels  mostly  filled  with  lymph,     x  1000. 

another  development  is  to 

be  seen  in  the  collection  of  a  number  of  these  larger  particles  of  chro- 
matin into  a  dense  mass  of  irregularly  round  outline  called  the  chro- 
matin knot.  In  this  case  the  chromatin  knot  is  always  placed  near  the 
periphery  of  the  nucleus  and  at  the  opposite  side  from  the  nucleolus 
(Fig.  157). 

These  various  forms  of  the  chromatin  particles  and  the  various  man- 
ners in  which  they  are  arranged  have  not  as  yet  given  us  any  generaliza- 
tions that  enable  the  nerve  tissue  to  be  better  understood.  They  show 
some  changes  during  the  operation  of  the  cell  and  in  certain  diseases. 

The  nucleolus  appears  in  the  nerve  cell  much  as  it  does  in  most  of  the 
other  cells  that  have  this  organ  well  developed.  It  is  nearly  always  large 
and  sometimes  very  large  (see  Figs.  160  and  161).  It  is  usually  an 
exact  sphere  as  far  as  the  eye  can  determine,  and  only  in  the  larger  forms 
is  any  irregularity  to  be  seen.  Like  other  nucleoli,  it  sometimes  contains 
one  or  more  "vacuoles,"  or  spaces  containing  non-staining  substances. 
Figure  161  shows  one  that  has  a  very  large  vacuole  containing  a  material 
which  shows  the  differentiation  of  a  chromatic  network.  Non-staining 
nucleoli  are  shown  in  the  sympathetic  ganglion  cells  of  the  muskrat 
(see  Fig.  158). 

The  whole  nucleus  is,  in  some  instances,  found  at  the  periphery  of 
the  cell  (see  Fig.  164)  and,  while  it  is  usually  single,  it  may  be  double 
in  some  cells  or  even  triple.  This  is  the  rule  in  the  cells  of  the  sym- 
pathetic ganglia  of  the  rodents  (Fig.  158),  and  a  frequent  exception  in 
the  electric  nerve  cells  of  the  torpedo  and  the  dorsal  cells  of  the  winter 
flounder's  spinal  cord  (see  Fig.  160). 

The  cytoplasm  of  a  neuron  and  its  processes  is  of  particular  interest 


182 


HISTOLOGY 


(I 


because  it  is  through  this  that  a  nerve  impulse  is  conducted.  While 
it  might  be  conceived  that  the  impulse  passed  through  the  entire  mass 
as  through  a  homogeneous  medium  (so  far  as 
the  impulse  was  concerned),  and  very  much  as 
an  electric  current  passes  through  a  copper  wire, 
yet  the  visible  presence  of  a  fibrillar  structure 
in  parts  of  the  cytoplasm  of  nerve  cells  drew 
attention  to  the  fact  that  here  were  nerve  fibrils 
that  perhaps  were  specific  cell-organs  of  conduc- 
tion. 

For  a  long  time,  while  it  was  recognized  that 
these  fibrils  might  form  such  paths,  it  could  not 
be  proved  that  any  one  or  more  of  them  ran  con- 
tinuously through  the  cytoplasm  for  any  distance 
that  would  warrant  regarding  them  as  such  or- 
gans. Apathy  and  other  investigators,  however, 
learned  how  to  stain  these  cells  in  such  a  man- 
ner that  particular  fibrils  were  differentiated  out 
of  the  mass  of  fibrillar  tissue  and  shown  to  run 
continuously  over  courses  that  were  more  than 
probably  the  same  as  those  taken  by  the  nerve- 
impulse.  Figure  159  shows  a  representation  of 
a  nerve  cell  prepared  by  Apathy.  His  method 
differentiated  out  several  kinds  of  fibrils.  At  present  it  is  known  that 
the  several  kinds  of  fibrils  thus  discovered  do  form  possible  paths  of 
conduction.  It  is  not  fully  demonstrated  that  these  paths  picked  out 
by  the  stain  are  the  only  ones.  It  is  probable  that  the  method  has 
picked  out  some  fibrils  physiologically  or  functionally  different  from  the 
others  just  as  Golgi's  method  selects  certain  neurons  to  the  exclusion  of 
others.  Figure  159  shows  a  cell  in  which  Apathy's  method  has  de- 
monstrated two  different  sets  of  fibrils,  one  of  which  is  supposed  by 
Apathy  to  bring  the  impulse  into  the  cell  and  the  other  to  take  it  out 
again. 

Seen  in  most  preparations,  the  cytoplasm  of  a  nerve  cell  is  ordinarily 
found  to  show,  besides  the  fibrils,  a  number  of  easily  seen  masses  of  a 
granular  substance  and  a  certain  proportion  of  undifferentiated  cyto- 
plasm, or  neuroplasnv,  as  it  should  be  called  in  this  case.  Besides  these 
usual  features  it  may  contain  pigment  bodies  and  other  rarer  structures, 
as  a  centrosphere,  centrosome,  cell-caps,  etc.  The  granular  substance 
stains  very  deeply  in  the  ordinary  staining  reagents.  Most  nerve  cells 
show  it  throughout  the  greater  part  of  their  cytoplasm  as  masses  of  a 
material  that  has  taken  the  stain  fully  as  deeply  as  the  chromatin  of  the 
nucleus,  while  all  other  cytoplasm  around  it  is  very  slightly  stained. 


FIG.  159.  —  A  small  nerve 
cell  from  the  medicinal 
leech,  stained  to  show 
the  two  sets  of  fibrils. 
(From  SCHNEIDER  after 
APATHY.) 


THE  NERVE    CELL 


183 


These  bodies  are  known  as  the  chromaphyllic  masses,  the  tigroid  bodies 
being  another  and  perhaps  more  convenient  name.  These  masses  are 
not  homogeneous,  but  are  composed  of  individual  granules  that  appear 
to  be  characteristic  organs  of  the  nerve  cell.  We  shall  call  the  granules 
neurochondria  to  distinguish  them  from  the  other  granules  that  are 
sometimes  found  in  the  nerve  cell. 

That  the  neurochondria  are  active  organs  of  nerve  work  may  be 
surmised  from  the  changes  that  take  place  in  them  under  different  physio- 
logical conditions.  Poisons  and  stimulants,  as  well  as  fatigue  and  disease, 
cause  marked  changes  in  their  amount  and  distribution.  They  are 
possibly  masses  of  food  material  for  the  trophic  processes  of  the  cell  or 


imp.  c. 


fev$^^>^>t  ^m^^Mii^^. 


FlG.  160.  —  Dorsal  giant  nerve  cell  from  cord  of  young  flounder,  Pseudopleuronectes.  Three 
nuclei.  The  chromophyllic  substance  in  two  forms,  a  finely  granular  deposit  in  the  cyto- 
plasm and  larger  spindle-shaped  bodies  in  both  cytoplasm  and  part  of  the  implantation 
cone.  Fibrillar  nature  of  implantation  cone  (imp.c.)  well  shown.  X  700. 

even  to  more  directly  support  the  nervous  activities  themselves.  Not  all 
nerve  cells  appear  to  possess  these  structures.  It  is  probable,  however, 
that,  considering  their  almost  universal  presence  in  the  various  nerve 
cells,  they  exist  in  these  apparent  exceptions  as  diffused  or  invisible 
structures. 

The  neurochondria  are  individually  so  small  as  to  be  almost  invisible. 
They  are  visible,  usually,  because  they  are  arranged  in  the  masses  spoken 
of  above,  the  tigroid  bodies  (see  Figs.  157  and  161).  These  bodies  are 
arranged  through  the  cytoplasm  (neuroplasm)  with  an  apparent  view  to 
meeting  two  conditions,  —  a  fairly  general  distribution  and  the  avoiding 
of  interference  with  the  courses  of  the  neuro-fibrils.  This  results  in  some 
such  figure  as  is  seen  in  Figures  157  or  161.  In  this  last,  which  is  a  nerve 
cell  from  the  cord  of  a  fish,  the  distribution  of  the  neurochondria  in 


1 84 


HISTOLOGY 


masses  would  be  clearly  shown  if  the  section  should  be  exposed  to  the 
action  of  trypsin,  when  the  other  cell  parts  would  be  digested  and  leave 
only  the  neurochondria,  which  could  then  be  studied  alone  and  apart 
from  any  of  the  other  cell-organs. 

Certain  of  the  tigroid  masses  are  remarkable  in  their  position  or  size. 
Some  of  them  form  a  cap-like  structure  on  one  or  more  sides  of  the 
nucleus.  Some  extend  out  into  the  dendritic  processes,  always,  however, 
becoming  narrower  and  longer  as  they  are  found  farther  from  the  cell 
body.  Their  absence  in  the  neurite  and  implantation  cone  must  be 


FIG.  161.  —  Giant  dorsal  nerve  cell  from  the  spinal  cord  of  the  marbled  angler,  Pterophryne 
histrio.     n.,  nucleus;  cy.,  cytoplasm;  ca.,  capillaries;  ca.n.,  nuclei  in  the  capillary  walls. 

looked  on  as  an  economy  of  space,  the  parts  in  question  being  too 
small  to  accommodate  the  tigroid  masses  which  can  just  as  well  be 
kept  in  the  cell  body. 

In  the  huge  dorsal  nerve  cells  of  the  young  of  the  winter  flounder  a 
peculiar  set  of  chromatic  bodies  make  their  appearance,  both  on  the  edge 
of  the  implantation  cone  and  extending  somewhat  out  into  the  fiber 
(Fig.  1 60).  These  are  probably  a  developmental  feature.  They  are  much 
larger  than  the  permanent  tigroid  masses  in  the  cell  and  are  very 
compact  and  smooth  in  outline.  They  possibly  are  reserve  stores  of  the 
chromaphyllic  substance  and  give  up  their  material  to  the  growing  and 
forming  tigroid  bodies.  Their  smooth  outline  and  compact  structure 
make  it  possible  that  they  belong  to  some  other  group  of  granules  in 
the  nerve  cell.  Such  essentially  different  granules  are  found  in  parts 


THE  NERVE    CELL 


IBS 


of  the  cytoplasm.     Some  are  fatty  in  nature  and  others  appear  to  be 

coagulated  lymph  or  blood  that  was  in  the  cell  at  the  time,  particularly 

in    the     channels 

that  are  described 

below. 

Many       nerve 

cells    are    among 

those  animal  cells 

that  approach  the 

limit  of  size  that  a 

cell  can  attain  and 

still  have  surface 

enough  to  perform 

its   nutritive    and 

excretory     ex- 
changes.     A  few 

go  above  that  size  ^^  '      ?  "  '-r';::    .^y^  >^\| 

and      in      conse-  ^^^v^^^=^  '  •  -f^"  ffr- 

quence  are  obliged 

to  develop  in  their 

cytoplasm  a  set  of 

channels  that  will 

serve  to  increase  this   power  of  exchange.     Among  these  are  many 

nerve  cells  that  possess  lymph  channels  or  spaces  of  various  degrees 

of  size  and  efficiency.  These  lymph  channels  can  be  seen,  weakly  de- 
veloped, in  the  sympathetic 
ganglion  cells  of  the  musk- 
rat  (see  Fig.  158),  and  more 
strongly  shown  in  the  larger 
nerve  cells  of  the  squid  (Fig. 
156),  in  which  latter  form 
they  also  are  occupied,  in 
part,  by  the  connective-tissue 
fibrils  that  penetrate  the  cell 
substance.  More  exceptional 
is  the  case  of  the  giant 
dorsal  nerve  cells  of  the  pe- 
diculate  fishes,  in  which  the 
capillaries  themselves,  with 
their  coat  of  connective- 
tissue  cells,  enter  the  cytoplasm  of  the  cell  and  supply  it  with  a  medium 

of  exchange  (Fig.  161). 

It  has  been  noted  above  that  connective-tissue  fibrils  enter  the  neuro- 


conn.i. 

FIG.  162. — Large  nerve  cells  from  sub-oesophageal  ganglion  of  Helix, 
conn.t.,  connective-tissue  elements  invading  cytoplasm;  gr.,  gran- 
ules in  the  cell  channels.  Xjoo.  (From  a  preparation  by  McCLURE.) 


FlG.  163.  —  Spinal  ganglion  nerve  cell  from  electro- 
cuted man.  pg.,  pigment  mass  lying  in  cytoplasm 
next  to  implantation  cone.  X  1000. 


1 86 


HISTOLOGY 


plasm  of  the  nerve  cell.  The  cells  that  produce  these  fibrils  are  some  of 
the  connective-tissue  elements  that  have  moved  into  the  nerve  tissue  and, 
in  connection  with  the  neuroglia  cells,  are  used  to  give  it  support  and 
tensile  strength.  In  some  of  the  lower  animals  they  form  a  thick  cover- 


FlG.  164.  —  Dorsal  nerve  cells  in  cord  of  a  flounder,  Achirus.     Shows  cytoplasmic  connection 
between  cells  and  an  eccentrically  placed  nucleus.     X  800. 

ing  for  the  cell,  and  their  fibrils  are  so  woven  into  the  outer  texture  of  the 
cell  that  they  are  with  difficulty  distinguished  from  the  neuro-fibrils 
except  by  special  staining  methods.  The  cell  bodies  of  these  connective- 
tissue  cells  may  lie  entirely  within  the  nerve  cell.  The  mollusk, 
Helix  (Fig.  162),  shows  such  conditions,  as  well  as  the  incidental  entrance 
of  connective  tissue  noted  above  in  the  pediculate  fishes  (see  Fig.  161) 
and  in  the  stellate  ganglion  cell  of  Loligo  (see  Fig.  156). 

Pigment  normally  appears  in  a  number  of  nerve  cells  as  a  collection 
of  small  brown  granules  occupying  some  particular  portion  of  the  cyto- 
plasm (Fig.  163).  This  is  of  unknown  use  to  the  cell.  In  disease  as  well 
as  in  old  age  the  amount  of  this  pigment  is  largely  increased. 

A  centrosphere,  sometimes  containing 
a  centrosome,  has  been  described  in  nerve 
cells  by  McClure,  Lewis,  and  others.  It  is 
possible  to  look  upon  these  structures 
either  as  vestigial  organs  left  from  the 
last  division  or  as  centers  of  some  present 
kinetic  operations  connected  with  the  activ- 
ities of  the  cell.  The  former  view  seems 
not  to  be  in  accord  with  what  we  know 
of  the  persistence  of  the  centrosome  after 
cell  division.  And  yet  we  shall  consider 
it  to  be  the  best  view  because  this  struc- 
ture is  only  occasionally  found,  even  in  the 
same  kind  of  cell.  Many  nerve  cells  show 
protoplasmic  processes  which  appear  to 
unite  with  similar  processes  from  other  cells.  This  is  well  illustrated 
by  the  "dorsal  nerve  cells"  from  the  cord  of  Achirus  lineatus  (Fig. 


FIG.  165. — Nerve  cell  from  the  brain 
of  a  lobster,  Homarus,  showing 
an  implantation  cone  (imp.c.)  that 
reaches  far  into  the  cell. 


THE  NERVE  FIBER  1 8? 

164).  Part  of  the  cell  body  is  sometimes  free  of  tigroid  bodies  and  is  of 
the  same  texture  as  the  nerve  fiber  of  which  it  is  a  direct  continuation. 
This  is  called  the  implantation  cone  in  the  vertebrates,  where  it  is 
roughly  cone-shaped  (see  Figs.  157,  158,  160,  and  161).  In  the  arthro- 
pods it  forms  a  long  curved  area  which  reaches  around  the  nucleus 
(Fig.  165). 

Technic.  — The  study  of  the  nerve-cell  bodies  requires  only  carefully 
prepared  paraffin  sections,  as  long  as  this  study  does  not  extend  to  the 
processes.  Staining  is  an  important  factor,  and  several  staining  methods 
have  been  evolved  for  the  purpose  of  learning  more  about  the  structure 
of  these  objects.  See  LEE'S  "  Microscopist's  Vade  Mecum." 

LITERATURE 

MANN,  G.  "The  Histology  of  Nerve  Cells,"  Report  of  the  68th  Meeting  of  the  British 
Association  for  the  Advancement  of  Science. 

ROHDE,  E.     "Die  Ganglienzelle,"  Zeits.  f.  Wiss.  Zool.,  Band  LXIV. 

McCLURE,  C.  F.  W.  "  On  the  Finer  Structure  of  the  Nerve  Cells  of  Invertebrates :  i  Gas- 
teropoda," in  Zool.  Jahrb.  Abt.  Morph.,  Band  XI,  1897. 


THE   NERVE  FIBER 

The  nerve  fiber  is  that  part  of  a  neuron  which  is  specialized  for  con- 
duction. It  is  an  integral  part  of  the  nerve  cell ;  an  evagination  of  its 
cytoplasm.  Its  growth,  or  the  process  of  its  evagination  from  the  cell, 
has  been  traced  in  the  living  embryonic  development  of  the  cell  and  in  the 
regeneration  of  nerve  tissue.  As  proved  by  experiment,  it  dies  when 
separated,  in  situ,  from  the  cell  body,  which  then  develops  a  new  and 
similar  process  to  take  its  place. 

Looked  upon  in  this  light,  we  can  see  that  its  structure  must  be  a 
modification  of  that  of  the  cell  body.  This  occurs  in  two  degrees,  forming 
two  kinds  of  fibers,  —  those  that  retain  the  tigroid  bodies  or  neurochon- 
dria  of  the  cell  body  and  those  which  do  not.  Some  dendritic  processes 
are  the  only  ones  that  retain  the  neurochondria,  and  they  do  this  for  only 
a  part  of  their  course.  Otherwise,  the  fiber  is  everywhere  the^same,  a 
continuation  of  the  cytoplasm  of  the  nerve  cell,  containing  a  bundle  of 
fibrils  that  are  either  continuations  of  those  in  the  cell  body  or  similar  to 
them.  The  fibrils  are  parallel  in  a  general  way,  and  as  has  been  stated 
in  discussing  the  nerve  cell,  they  are  probably  structures  by  whose  agency 
the  nerve  impulse  is  forwarded  through  the  neuron.  That  the  fiber  can 
forward  the  impulse  from  perceptory  surface  to  discharging  surface 
without  the  direct  aid  of  the  cell  body,  or  even  in  the  absence  of  the  cell 
body,  is  proved  by  the  experiments  of  Bethe  on  certain  of  the  nerve  cells 
of  a  crab.  These  cells  were  selected  for  the  experiments  because  of  the 


1 88  HISTOLOGY 

4 

fact  that  the  efferent  and  afferent  fibers  both  arose  from  a  single  process 
of  the  cell  in  such  a  way  that  the  impulse  paths  did  not  lie  in  the  main 
cell  body  at  all,  but  passed  directly  from  the  dendrite  to  the  neurite.  This 
process,  bearing  the  two  fibers,  was  cut  off  and  the  cell  body  removed 
from  the  tissue  without  disturbing  the  connection  between  the  fibers  or 
disturbing  their  relations  to  the  other  tissues  in  any  way.  Under  these  cir- 
cumstances the  nerve  continued  to  carry  the  impulse  as  before,  and  kept 
up  its  usual  function  until  its  death  in  the  otherwise  uninjured  tissues. 
Its  death  was  undoubtedly  due  to  the  cutting  off  of  its  nutrition  and  other 
trophic  benefits  formerly  derived  from  the  cell  body  with  its  nucleus. 

This  latter  fact  has  also  been  observed  in  the  pathological  and  ex- 
perimental cutting  off  of  masses  of  fibers  from  their  nerve  cells.  In  these 
cases  the  fibers  die  and  new  processes  grow  out  from  the  same  cells  to  take 
their  places. 

The  nerve  fiber  is  a  structure  that  is  only  called  into  existence  when 
either  the  perceptory  or  the  motor  surface  of  the  cell  is  situated  at  a  dis- 
tance from  the  cell  body.  Consequently  its  length  is  variable  and  in 
some  cases  is  reduced  to  the  length  of  an  ordinary  cell. 

The  neurite,  as  has  been  previously  said,  leaves  the  cell  body  from  an 
implantation  cone,  which  is  the  intermediate  portion  between  the  cell  body 
and  the  fiber.  This  structure  may  vary  from  the  short  conical  form  seen 
in  most  nerve  cells  to  the  more  extensive  kinds  that  appear  in  some  of  the 
invertebrate  nerve  cells,  as,  for  instance,  the  lobster,  where  the  implanta- 
tion cone  is  narrowed  to  the  diameter  and  continues  as  an  extension  of  the 
fiber  itself  into  the  cell,  forming  a  long  curved  path  that  sometimes 
encircles  the  nucleus  before  its  substance  merges  with  that  of  the  cyto- 
plasm and  its  fibrils  are  no  longer  to  be  distinguished  among  the  neuro- 
chondria  (see  Fig.  165).  Also  see  other  figures  under  nerve  cells. 

There  are  interesting  accessory  tissues  found  in  connection  with  the 
nerve  fibers  and  used  to  provide  them  with  coverings  for  their  protection, 
and  with  support  and  union  in  their  common  pathways,  the  nerve  tracts. 
The  latter  of  these  are  the  neuroglia  cells,  and  will  be  treated  of  in  the 
next  part  but  one.  The  former  are  the  true  connective-tissue  cells  that 
form  the  sheaths  covering  most  nerve  fibers  (Fig.  166). 

These  sheath  cells  have  been  thought  to  be  of  ectodermal  origin  and 
to  have  migrated,  along  with  the  nerve  process,  to  the  positions  in  which 
they  are  found.  It  has  been  proved,  however,  that  they  are  true  connec- 
tive-tissue cells  from  the  locality  through  which  the  nerve  fiber  has  passed 
in  its  development.  They  may  be  compared  closely  with  the  connective- 
tissue  cells  that  surround  some  of  the  nerve-cell  bodies  in  the  ganglia. 
A  nerve  fiber  is  not  always  provided  with  this  covering.  Some  few  kinds 
have  none  whatever  other  than  the  unspecialized  connective  or  other 
tissue  through  which  they  pass.  The  simplest  form  in  which  a  definite 


THE  NERVE  FIBER 


189 


covering  is  found  is  one  in  which  a  single  layer  of  these  sheath  cells 
have  united  by  their  edges  to  form  an  unbroken  tubular  sheath  some- 
times called  the  neurolemma.  The  nuclei  of  these  cells  appear  as  if  they 
lie  on  the  outside  of  the  sheath.  They  do  not  do  this,  however,  as  the 
cell-substance  can  be  seen,  with  higher  powers,  covering  the  nucleus 
as  is  done  in  all  cells. 

In  the  higher  vertebrate  animals  there  is  found  on  some  nerve  fibers, 
in  addition  to  this  connective- tissue  sheath,  another  and  inner  covering 
of  an  entirely  different  nature.  This'  is  a  layer  of  a  thick  oily  or  fatty 


FIG.  166.  —  A,  two  non-medullated  nerve  fibers  from  a  large  connective  in  the  Octopus.  The 
left-hand  fiber  is  a  section.  That  on  the  right  is  a  surface  view  showing  the  delicate,  branch- 
ing connective  cell  which  covers  it.  Fibrillation  not  shown.  B,  a  group  of  nerve  fibers  and 
parts  of  two  other  groups  in  the  developing  optic  nerve  of  a  three-months-old  human  embryo. 
X  1000. 

substance  called  myelin.  It  forms  a  complete  layer  around  parts  of  the 
fiber  only  (Fig.  167).  As  to  its  origin,  its  position  between  the  nerve 
process  and  the  surrounding  sheath  leaves  it  to  be  decided  as  to  which 
of  these  two  have  produced  it.  As  the  neurite  is  a  highly  specialized  struc- 
ture which  is  probably  expending  all  of  its  energies  in  the  work  of  conduc- 
tion, it  is  probable  that  the  other,  the  sheath,  is  the  producer  of  the  mye- 
lin, and  that  it  secretes  it  much  as  some  other  connective-tissue  cells,  the 
fat-cells,  secrete  the  fat,  by  the  metabolic  activity  of  their  cytoplasm. 
Whether  this  secretion  is  a  superficial  one,  leaving  the  myelin  between 
the  inner  surface  of  the  sheath  and  the  outer  surface  of  the  neurite,  or 
whether  it  is  an  intra-cellular  secretion  of  the  sheath  cell  in  which  each 


ICp  HISTOLOGY 

cell  forms  the  myelin  in  its  interior,  thus  leaving  a  thin  layer  of  cytoplasm 
between  the  myelin  and  the  nerve  process  or  neurite,  is  yet  to  be  decided. 
The  first  view  is  favored  by  the  apparent  absence  of  any  structure  (cyto- 
plasm) lying  between  the  myelin  and  the  neurite.  The  second  possibility 
is  advocated  by  some  because  of  the  presence  of  the  nodes  which  separate 
the  myelin  of  the  two  consecutive  sheath  cells  as  if  it  were  an  internal 
product.  The  usual  presence  of  but  one  sheath-nucleus  between  any  two 
neighboring  nodes  heightens  this  latter  probability. 

The  myelin  exists  as  a  fine  emulsion  and  is  not  homogeneous,  but  lies 
in  the  interstices  of  a  fine  network  of  a  substance  that  resembles  keratin 
or  horn.  This  material  gives  off  an  odor  like  that  of  horn  or  feathers 
when  it  is  freed  from  the  myelin  and  burnt. 

As  has  been  intimated,  the  myelin  sheath  is  not  continuous.  At 
various  distances,  long  when  compared  with  the  fiber's  diameter,  it  is 
constricted  and  separated  by  the  substances  of  the  sheath,  thus  forming 
the  nodes.  There  is  discussion  as  to  whether  these  nodes  or,  indeed,  the 


FIG.  167.  — Portion  of  a  medullated  nerve  fiber  from  a  mammal.     Node  shown  near  middle. 
Incisure  shown  near  left  end.     X  1000. 


sheath  itself  is  to  be  found  in  those  medullated  fibers  that  pass  through 
the  brain  and  spinal  cord  of  the  mammals.  They  are  probably  provided 
with  a  sheath,  as  this  appears  in  their  development.  Its  apparent  ab- 
sence in  the  adult  form  is  probably  due  to  its  extreme  delicacy,  there  being 
no  need  of  a  strong  and  substantial  sheath  on  the  fiber  in  this  position  on 
account  of  the  strong  and  heavy  surroundings. 

Where  many  fibers  run  in  the  same  course  they  are  usually  found 
together,  forming  a  nerve.  This  collection  of  fibers  is  held  together  by 
a  connective-tissue  covering,  the  endoneurium,  and  several  of  the  smaller 
bundles  are  often  found  to  be  surrounded  by  a  still  thicker  and  stronger 
covering  of  the  same  kind  of  material.  Such  a  composite  bundle  is  also 
called  a  nerve,  and  its  covering  a  perineurium. 

Technic.  —  Fibers  of  all  kinds  may  be  fixed  and  teased  and  examined 
individually  for  both  general  features  and  to  see  the  fibrillar  nature  of  the 
nerve  process.  A  fiber  cannot  be  traced  for  any  distance  in  this  way, 
however,  but  must  be  stained  by  the  Golgi  method  or  the  methylene-blue 
method.  As  the  first  of  these  stains  can  be  made  to  have  a  selective 
action  and  to  pick  out  only  one  or  a  few  fibers  from  the  great  mass  that 
exist  in  most  of  the  fiber  paths  or  nerves,  it  is  the  most  useful  method 


MOTOR  NERVE-ENDING  IQI 

known  in  working  out  the  fiber  details  of  ganglion  structure.  The 
second,  or  methylene-blue  method  is  used  more  to  follow  the  fiber 
courses  near  the  periphery  and  the  nerve  elements  in  the  sense  organs. 
Both  methods  are  capricious  in  their  results  and  can  be  made  to  suc- 
ceed only  by  constant  effort  and  as  the  result  of  experience.  One  must 
be  prepared  to  have  them  fail  one  time  after  another  and  yet  to  expect 
good  results  the  next  time.  No  one  of  the  numerous  variations  will  be 
set  out  at  length  here,  but  the  student  must  have  the  individual  direc- 
tion of  an  instructor  and  work  up  the  particular  modifications  that 
he  finds  most  satisfactory  from  among  the  many  that  are  described 
in  LEE'S  "  Microscopist's  Vade  Mecum." 

LITERATURE 

Besides  the  parts  devoted  to  the  nerve  fiber  in  Schneider's,  Barker's,  and  other  text- 
books, it  is  enlightening  to  read  the  following:  — 

HARRISON,  R.  G.  "Observations  on  the  Living  Developing  Nerve  Fiber,"  Proc.  of  the 
Soc.  for  Experimental  Biology  and  Medicine,  1907,  pp.  140-143. 

NEAL,  H.  V.  The  "Development  of  the  Ventral  Nerves  in  Selachii,"  Mark  Ann.  Vol- 
ume, 1903. 

APATHY,  S.  "Das  leitende  Element  dcs  Nervensystems  und  seine  Beziehungen  zu  den 
Zellen,"  Mitt.  a.  d.  Zool.  Sta.  zu  Neapel,  Vol.  XII,  1897,  p.  495. 

BARDEEN,  C.  R.  "The  Growth  and  Histogenesis  of  the  Cerebro-spinal  Nerves  in  Mam- 
mals," Am.  Journ.  of  Anat.,  1902,  Vol.  XI,  p.  231. 

RETZIUS,  G.     "Was  ist  die  Henlesche  Schide  der  Nerve nfasern?"  Anat.  Am.,  Band  XV. 

WYNN,  W.  H.  "The  Minute  Structure  of  the  Medullary  Sheath  of  Nerve  Fibers,"  Jour- 
nal of  Anatomy  and  Physiology,  Vol.  XXXIV. 


THE   MOTOR    END-ORGANS   OF   NERVE   CELLS 

By  motor  end-organ  is  meant  the  modification  of  the  end  of  the  effer- 
ent process  of  a  nerve  cell  which  enables  it  to  transfer  or  discharge  its 
impulse  as  a  stimulus  to  another  nerve  cell  or  to  a  muscle  cell,  an  electric 
cell,  or  a  gland  secreting  cell.  Such  an  impulse  is  used  to  discharge  the 
secretion  material  or  to  cause  it  to  be  used  in  situ. 

While  differing  to  some  degree  in  size  and  complexity  among  them- 
selves, the  several  kinds  of  motor  end -organs  do  not  present  flic  great 
variety  of  form  and  adaptation  that  the  end -organs  on  the  other  or  per- 
ceptory  pole  of  the  nerve  cell  do.  Nor  can  we  distinguish  any  specific 
cell-organs  pertaining  to  the  particular  function  of  each  of  the  organs 
as  we  can  in  the  perceptory  endings,  especially  those  that  are  situated  in 
the  periphery. 

As  in  the  perceptory  ending,  the  motor  endings  may  be  placed  directly 
on  the  nerve -cell  body  or  removed  from  it  to  the  end  of  one  of  its 
processes,  the  efferent  fiber.  Like  the  fiber,  it  is  an  integral  part  of  the 
cell,  a  direct  continuation  of  the  fiber  itself.  The  exact  relations  of  the 


192 


HISTOLOGY 


neuro-fibrils  of  the  nerve  cell  and  fiber  to  the  motor  endings  are  not 
known  other  than  that  they  arc  direct  continuations  of  one  or  more  of 
the  fibrils  which  become  thicker  and  separated  into  a  varying  number 
of  short  branches  that  develop  varicosities  and  an  irregular  outline. 
The  terminal  varicosities  of  the  various  branches  sometimes  attain  con- 
siderable size,  as,  for  instance,  the  end-organs  of  some  electric  nerves 
(Fig.  1 68).  In  such  a  case  the  sarcoplasm  or  neuroplasm  probably 


FIG.  168.  —  A  nerve  fiber  (nv.f.)  ending  on 
an  electroplax  of  Astroscopus.  The  end- 
plate  is  large  and  stained  black  with  silver 
nitrate. 


FIG.  169. — -A  Purkinje  cell  from  the  cerebellum 
of  man,  showing  the  contact  of  two  different 
motor  end-organs  of  nerve  cells  with  its  sensory 
surfaces,  cl.fi.,  ending  of  a  climbing  fiber  on 
the  branching  dendrites;  stell.end.,  ending  of  a 
stellate  fiber  on  a  surface  of  the  cell  body. 
(AfterCAjAL;  slightly  modified.) 


constitutes  some  considerable  part  of  the  structure  in  addition  to  the 
neuro-fibrils. 

The  terminal  branches  or  telodendria  of  the  end-organ  are  plainest 
and  least  irregular  as  well  as,  usually,  most  thickly  branched  in  case 
they  are  used  to  communicate  with  another  nerve  cell. 

Examples  of  efferent  end -organs  in  contact  with  other  nerve  cells 
or  their  processes  are  most  numerous.  The  simplest  type  of  such  an 
organ  is  represented  by  a  sensory  nerve  cell,  as,  for  instance,  the  auditory 


MOTOR  NERVE-ENDING 


193 


or  tactile  cell  of  a  vertebrate,  without  any  processes  and  with  its  discharg- 
ing surface  directly  in  contact  with  the  afferent  or  perceptory  end-brush 
of  a  fiber  leading  to 
the  central  nervous 
system  (see  Fig.  198). 
The  most  fre- 
quent type  is  exem- 
plified, perhaps,  by 
the  connection  estab- 
lished between  the 
discharging  end  of  a 
branch  of  a  stellate 
nerve  cell  of  the  cer- 
ebellum with  the  cell 
body  of  one  of  the 
Purkinje  cells  in  the 
same  organ.  This 
relation  is  shown  in 
Figure  169,  stell.  end., 
but  its  value  as  an 
example  is  impaired 
by  the  fact  that  the 
cell-body  surface  of 
the  Purkinje  cell  is 


olf. 


FIG.  170.  —  Portion  of  a  section  of  the  olfactory  bulb  of  man. 
Stained  with  nitrate  of  silver  to  show  the  perceptory  endings  of 
mitral  cells  (mi.c.)  in  contact  with  the  stimulatory  end-organs  of 
the  olfactory  cells,  which  are  not  shown  in  the  figure,  but  whose 
efferent  fibers  enter  the  figure  from  the  sides  (olf.fi.).  The  oval 
area  in  which  this  meeting  takes  place  is  called  an  olfactory 
glomerulus  (olf.gl.).  (From  HUBER  after  GOLGI  and  CAJAL.) 


not  its  main  per- 
ceptory surface,  this 
being  represented  by 

the  widely  branching  dendrites,  which  furnish  us  with  an  example 
of  another  and  more  typical  variation  of  the  same  kind  of  connec- 
tion as  that  mentioned  last.  This  is  the  contact  of  the  discharging 
end  of  a  nerve  fiber  (here  the  cerebellar  climbing  fiber)  with  the  percep- 
tory processes  of  the  Purkinje  cell  (Fig.  169,  cl.fi.}.  The  finely  branched 
fibers  of  the  two  end  -organs  lie  parallel  with  one  another  and  in  a_contact 
that  is  sufficiently  extensive  and  intimate  to  permit  of  the  nerve  impulse 
being  transferred  from  one  to  the  other.  Thus,  a  neuron  may  have  two 
perceptory  surfaces. 

A  last  example  of  such  a  connection  is  shown  by  the  mingling  of  the 
discharging  tclodendron  of  an  olfactory  nerve  cell  of  a  mammal  with 
the  perceptory  branches  on  the  end  of  an  afferent  fiber  leading  to  one  of 
the  mitral  cells  of  the  olfactory  lobe  of  the  brain  (Fig.  170). 

The  discharging  end-organs  of  nerves  that  serve  to  stimulate  muscle 
fibers  into  motion  are  well  known,.  and  two  or  three  examples  will  give  a 
clear  conception  of  their  structure.  The  simplest  form  is  undoubtedly 


194 


HISTOLOGY 


that  seen  in  the  innervation  of  some  of  the  smooth  muscle  fibers  of  the  in- 
vertebrate animals.  The  leech  has  shown  such  an  organ  on  some  of  the 
muscle  cells  in  the  circular  layer  of  muscle  tissue  in  its  body  wall  (Fig. 


FIG.  171.  —  Stippled  outline  of  part  of  a  muscle  fiber  from  the  alimentary  canal  of  the  leech, 
showing  the  end-organ  of  a  motor  nerve  in  contact  with  it.     (From  SCHNEIDER  after  APATHY.) 

171).     Here  the  innervating  fiber  branches  into  simple  and  smooth  divi- 
sions that  apply  themselves  to  the  body  of  the  muscle  cell. 

It  has  been  questioned  if  these  smooth  terminal  branches  really  rep- 
resented the  end-organs,  and  suggested  that  the  real  end-organs  had  failed 
to  take  the  stain,  thus  remaining  invisible  in  the  preparation.  Huber 
has  found  in  the  smooth  muscle  of  the  cat  that  the  fibers  do  end  in  small 
but  distinct  varicosities. 


FIG.  172.  —  A,  motor  nerve  end-organ  on  portions  of  two  voluntary  muscle  fibers  of  Lacerta. 
(From  BOHM  and  DAVIDOFF'S  Histology.)  B,  motor  nerve  end-organs  on  striated  muscle 
fibers  of  the  frog.  (After  SIHLER  in  Zeits.f.  wiss.  Zool.) 


A  more  specialized  motor  nerve-ending  on  muscle  can  be  seen  in  the 
nerve-ending  on  amphibian  muscle  tissue  (Fig.  172,  B}.  The  motor 
ending  on  the  striated  muscle  fiber  of  the  frog  is  branched  into  rather 
long  but  decidedly  thick  and  more  irregular  divisions.  These  show, 
also,  a  feature  characteristic  of  the  more  highly  developed  motor  muscle 


MOTOR  NERVE-ENDING  1 95 

endings  in  general,  the  gathering  of  the  undifferentiated  muscle  cyto- 
plasm, or  sarcoplasm,  around  the  end-organ.  The  whole  mass,  partly 
nerve  substance  and  partly  muscle  substance,  is  included  when  we  use 
the  term  end-plate. 

A  further  complexity  and  development  is  seen  in  the  motor  end-organs 
of  the  reptiles  and  the  higher  vertebrates.  The  lizard  shows  excellent 
examples  (Fig.  172,  .4).  The  end  branches  of  the  nerve  or  telodendria 
are  short  and  thick  and  very  irregular.  They  anastomose  to  form  a 
thick,  heavy  plexus.  The  collection  of  sarcoplasm  around  the  plexus 
so  formed  is  very  large  (see  also  the  part  on  the  muscle  fiber).  (Also 
read  the  parts  on  the  nerve-endings  on  the  different  electric  cells.) 

The  efferent  processes  of  the  nerve  cells  also  end  on  gland  cells.  They 
thus  transmit  an  impulse  to  the  gland  cell  to  secrete.  Whether  the  im- 
pulse is  necessary  to  the  metabolism  of  the  food  materials  into  the  secre- 
tion or  whether  it  merely  excites  the  cell  to  discharge  or  prepare  to  dis- 
charge the  secretion  already  prepared  by  its  cytoplasm  is  not  definitely 
known.  The  fact  that  many  cells  secrete  apparently  without  any  nerve 
supply  would  point  to  the  latter  view  in  our  example.  In  controlling 
the  metabolism  itself  the  nerve  cell  would  only  be  doing  what  it  prob- 
ably does  in  muscle  cells  and  others.  One  must  also  be  careful  to 
distinguish  such  endings  as  motor  in  character,  because  sensory  end- 
ings are  found  in  so  many  positions  on  the  periphery. 

The  manner  of  ending  in  gland  cells  is  well  shown  in  the  nerve-ending 
in  the  tear  gland  of  the  rabbit  (Fig.  173).  Fibers  from  several  sources 
form  plexes  around  the  alveoli  of  this  gland,  and  their  terminal  branches 
enter  the  glandular  epithelium.  Here  they  form  motoj  endings  that  are 
pressed  against  the  proximal  and  middle  surfaces  of  the  gland  cells. 
These  terminal  fibers,  which  are  the  motor  end-organs,  differ  from  the 
fiber  from  which  they  come  only  in  that  they  are  irregular  in  contour  and 
have  several  thickened 
portions,  the  varicosi- 
ties,  of  which  a  num- 
ber occur  somewhat 
regularly  distributed 
on  each  branch.  The 

variCOSltieS   are    about  FI(J   I73._Stimulatory  nerve  end-organs  in  the  epithelium 

twice    the   diameter  of  of  the  lachrymal  gland  of  a  rabbit.    A,  lateral  view,  with 

the  fiber  and  are  nearly  Basement  membrane   (b.m.)  •  B,   superficial  view.      X  200. 

*  (After  A.  DOGIEL  m  Arch.  f.  mik.  Anat.) 

spherical  in  shape. 

These  nerves  are  not  the  only  ones  that  direct  the  activities  of  the  gland. 
Others  act  indirectly  through  the  medium  of  the  muscle  fibers  that 
regulate  the  blood  supply  and  those  that  act  as  compressors  of  the  gland 
mass  itself. 


196 


HISTOLOGY 


A  widely  different  type  of  gland,  the  nephridium,  also  has  motor  nerve- 
endings  which  control  the  activities  of  its  epithelial  cells.  Nerve  fibers 

have  been  found  which 
all  but  end  in  the  renal 
cells  of  the  earthworm. 
But  little  effort  with 
methylene  blue  would 
suffice  to  bring  out  the 
terminal  organ  which 
must  be  closely  applied 
to  or  actually  entered 
into  the  cytoplasm. 

Figure  1 74  shows  this 
(After  SMIRNOW  state  as  actually  demon- 
strated in  the  nerve- 
endings  in  the  frog's  renal  epithelium.  The  nerve  fibrils  here  pass 
into  the  cytoplasm  as  varicose  fibrils  with  an  irregular  course  and  in 
sufficient  number  to  control  the  activities. 

Technic.  — Methylene  blue  is  the  best  method  for  studying  these 
structures.  Nitrate  of  silver  will  sometimes  give  good  results,  but  the 
infra  vitam  methylene-blue  method  will  satisfy  all  needs  when  once  the 
investigator  has  mastered  it  and  adapted  it  to  his  needs. 


FIG.  174.  —  Portion  of  renal  epithelium  of  frog,  showing 
stimulatory  nerve-endings  in  the  cells, 
in  Zool.  Anz.) 


LITERATURE 

RETZITJS,  G.     "Zur  Kentniss  der  motorischen  Nervenendigungen,"  Biol.  Unters.,  1892. 
HUBER,  G.  C.,  and'DE  WITT,  L.     "A  Contribution  to  the  Motor  Nerve-Endings,"  etc., 

Journ.  of  Cotnp.  Neurology,  Vol.  VII,  1897,  p.  169. 
SIHLER,  CHR.     "Neue  Untersuchungen  uber  die  Nerven  der  Muskeln  mit  besonderer 

Berucksichtigung  umstrittener  Fragen,"  Zeit.fur  Wiss.  Zool.,  Band  LXVIII,  1900. 


NEUROGLIA 

The  neuroglia  tissue  is  the  supporting  tissue  of  the  nervous  system  that 
has  been  derived  from  the  ectodermal  cells  in  the  early  history  of  the  devel- 
opment of  the  individual.  It  is  derived  from  the  same  layer  of  cells  that 
the  nerve  cells  themselves  are,  and  its  relations  can  be  made  clear  by  looking 
at  a  section  of  the  spinal  cord  of  a  fish  where  the  ependymal  cells,  which 
are  neuroglia  cells,  show  a  condition  that  is  halfway  between  a  purely  epi- 
thelial form  and  the  internal  neuroglia  cells  found  in  the  brain,  where  they 
have  assumed  a  form  that  would  lead  one  ignorant  of  their  origin  to 
call  them  connective-tissue  cells.  Figure  175  represents  four  neuroglia 
cells  that  are  graded  with  reference  to  their  degree  of  specialization  as  to 
branching  and  internal  position.  This  series  is  neither  ontogenetic  or 


NEUROGLIA 


197 


taxonomic,  but  a  combination  of  both.  It  serves,  however,  to  indicate 
the  growth  of  the  cell  away  from  the  surface,  the  moving  of  its  nucleated 
body  toward  its  internal  or  proximal  end,  and  its  final  separation  from 
all  connection  with  the  surface.  The  successive  stages  of  branching  as 
the  cell  moves  away  are  also  easily  noted. 


FIG.  175.  —  Four  neuroglia  cells  taken  from  different  sources  to  show  four  grades  of  specializa- 
tion as  connective  cells  of  the  nerve  tissues.  The  specialization  consists  of  increasing 
removal  from  the  surface  and  the  development  of  branches.  Individual  figures  taken  from 
"  STOHR'S  Text-book  of  Histology  "  by  LEWIS. 

The  relation  is  clearer  here  than  in  any  of  the  invertebrate  tis- 
sues, where,  however,  the  same  truth  holds  for  neuroglia  as  to  its 
origin  and  use. 

The  most  characteristic  struc- 
tural feature  of  neuroglia  tissue  is 
the  rather  small  number  of  smooth, 
strong  fibrils  that  the  branching 
cytoplasmic  processes  of  the  cells 
produce.  These  fibrils  are  unlike 
the  fibrils  of  simple  binding  con- 
nective tissue  in  that  they  are  long 
and  of  a  uniform  size  and  a  smooth, 
even  contour.  They  do  not  branch 
as  a  rule,  and  are  not  produced  by 
the  cytoplasm  of  the  central  mass, 
but  rather  by  the  peripheral  por- 
tions and  its  processes.  The  dense 
branching  effect  of  the  neuroglia 
cell,  when  stained  by  the  Golgi  process,  is  due  to  the  staining  of  the 
protoplasm  in  which  the  fibrils  lie,  as  well  as  the  fibrils  themselves. 


FIG.  1 76.  —  Part  of  a  section  of  the  spinal  cord 
of  a  rabbit,  gl.c.,  neuroglia  cell;  fi.,  neurog- 
lia fibrils;  conn.  I.e.,  connective  tissue  cell. 
(After  K.  C.  SCHNEIDER.) 


198 


HISTOLOGY 


But  few  stains  will  properly  differentiate  these  fibrils  from  the  cyto- 
plasm of  the  cells  and  from  other  kinds  of  fibers.     Figure  176  shows 

this  separation  effected  by  means  of 
the  "Mallory  "  stain. 

These  cells  must  not  be  con- 
founded with  the  real  connective- 
tissue  cells  of  mesodermal  origin  that 
are  found  in  the  nerve  tissue.  Such 
cells  are  especially  prominent  near 
the  blood  vessels  and  can  easily  be 
distinguished  from  neuroglia  in  many 
ways.  Figure  176  shows  a  compari- 
son between  the  two  kinds.  Note 
the  finer  and  more  numerous  as  well 
as  more  lightly  stained  fibrils  of  the 
connective  tissue;  also  the  nucleus, 
which  is  far  smaller  in  the  neurog- 
lia cells. 

In  the  invertebrate  animals  are 
found  all  stages  of  specialization  of 
the  neuroglia  cell  from  the  embryonic 
neuroblast.  The  fully  adult  forms, 
which  will  alone  be  shown  here,  usu- 
ally have  some  portion  of  their  body 
on  the  surface  of  the  nerve  center 
or  nerve  cord  in  which  they  lie,  thus 
indicating  their  origin  from  the  embryonic  surface  of  the  body,  of  which 
most  of  the  nerve  structures  were  formerly  a  part.  Figure  177  shows  a 
portion  of  a  nerve  cord  of  a  gasteropod  mollusk,  Sycotypus,  cut  in  trans- 
verse section.  The  neuroglia  cells,  here  ependymal  in  form,  are  situ- 
ated with  their  principal  cytoplasmic  masses,  containing  the  nucleus, 
on  the  periphery,  and  the  branching  cytoplasm,  containing  the  fibrils, 
extends  toward  the  center  of  the  nerve  cord  and  branches  freely  to  form 
a  support  for  the  numerous  bundles  of  nerve  fibers  running  through  it. 

A  more  involved  form,  of  great  specialization,  is  to  be  seen  in  the  enor- 
mous neuroglia  cells  found  scattered  in  some  of  the  nerve  cords  of  the 
common  leech,  Hirudo  medicinalis.  This  huge  cell  is  placed  in  the  center 
of  the  nerve  cord  and  sends  branching  processes  outward  to  the  surface. 
The  neuroglia  fibrils  are  contained  in  these  processes,  which  produce  and 
maintain  them.  The  processes,  here  as  before,  act  as  a  supporting  scaf- 
fold for  the  nerve  fibers  that  pass  through  the  cord.  They  also  flatten 
against  the  surface  of  the  cord  to  which  they  are  fastened  (Fig.  178). 
Although  these  large  cells  are  more  than  probably  derived  from  the 


FIG.  177.  —  Neuroglia  cells  and  their 
branches  in  a  nerve  cord  of  the  gasteropod 
mollusk  Sycotypus.  neu.c.,  neuroglia  cells. 


NEUROGLIA 


199 


surface  of  the  cord,  and  thus  from  the  epithelium  of  the  embryonic  body, 
the  exact  method  of  their  development  and  growth  into  this  secondary 
position  has  not  been  worked  out.  Neu- 
roglia  cells  show  a  strong  tendency  to 
syncytial  arrangement,  due,  as  in  connec- 
tive tissue,  to  their  function. 

Neuroglia  is  found,  not  only  in  the 
central  nervous  system  and  nerve  cords 
of  most  animals,  but  also  in  the  sensory 
nerve  tissues  on  the  periphery.  Its  most 
primitive  form  can,  perhaps,  be  seen  in 
the  "  supporting  cells  "  of  olfactory  and 
tactile  epithelia.  Here  the  cells  have  no 
processes,  and  little  resemble  neuroglia 
elements,  and  it  is  only  by  virtue  of 
their  association  with  sensory  nerve  cells 
that  they  can  be  called  neuroglia  at 
all. 

In  the  visual  tissues  these  cells  show 
a  distinct  advance  as  neuroglia  cells 
over  the  condition  found  in  the  other 
sense  organs.  This  advance  is  not 
marked  in  the  eyes  of  the  lower  invertebrates.  A  mere  beginning 
is  represented  in  the  glia  cells  found  in  the  eyes  of  planarian  worms. 
In  the  vertebrate  visual  tissue,  the  retina,  neuroglia  is  well  developed, 
almost  as  much  so  as  in  the  central  nervous  tissues.  This  is  easily  under- 
stood when  we  recall  that  the  retina  is  only  secondarily  derived  from  the 
surface,  being  evaginated  from  the  brain,  which  was  in  its  turn  invagi- 
nated  from  the  dorsal  surface  of  the  young  embryo.  The  retinal  neu- 
roglia cell,  or  radial  fiber,  as  it  is  called,  is  considerably  branched,  and 
acts  as  an  efficient  support  to  the  delicate  nerve  cells  of  the  retina,  from 
whose  embryonic  ancestral  cells  it  was  also  derived  (see  Fig.  175,  B). 

Technic.  — The  general  outlines  of  neuroglia  cells  may  be  most  satis- 
factorily seen  when  the  tissue  has  been  prepared  by  one  of  the  methods 
of  Golgi.  After  the  location  of  these  kinds  of  cells  is  known,  their  cell 
bodies  may  be  recognized  in  many  ordinary  preparations  and  distin- 
guished from  the  ordinary  connective-tissue  cells  in  that  preparation. 
It  may  be  distinguished  with  certainty  from  these  cells  in  two  ways: 
by  the  use  of  special  stains,  as  Mallory's  stain  for  neuroglia,  or  by  a  study 
of  its  histogenesis.  This  latter  way  of  determining  the  nature  of  the  cell 
is  most  satisfactory  and  is  not  difficult. 


FIG.  1 78.  —  Transaction  of  a  nerve  con- 
nective in  the  leech  to  show  one  of  the 
large  central  neuroglia  cells  with  its 
fibrillar  processes  reaching  out  in  all 
directions  to  the  periphery  and  act- 
ing as  a  support  for  the  numerous 
nerve  fibers  that  run  between  them 
in  bundles.  (After  K.  C.  SCHNEIDER.) 


200  HISTOLOGY 


LITERATURE 

HARDESTY,  I.      "The  Neuroglia  of  the  Spinal  Cord  of  the  Elephant,"  Am.  Journ.  of 

Anal.,  1902-1903,  Vol.  II,  p.  81. 
HUBER,   G.  C.     "Studies  on  Neuroglia  Tissue,"  etc.,  Contrib.  Med.  Research,  ded.  to 

V.  C.  Vaughan,  Ann  Arbor,  1903,  p.  578. 

JOSEPH,  G.     "Zur  Kentniss  der  Neuroglia,"  Anal.  Anz.,  Band  XVII. 
WAWKZIK,   E.     "Uber  das  Stutzgewebe  des  Nervensystems  der   Chaetopoden,"   Zool. 

Beitrage,  Band  III,  1892. 
HATAI,  SHINKISHI.     "  On  the  Origin  of  Neuroglia  Tissue  from  the  Mesoblast,"  Journ. 

Comp.  N enrol.,  Vol.  XII,  No.  4,  1900. 


TISSUES   OF  TOUCH,   OR   TACTILE  TISSUES 

The  simplest  form  of  stimulus  that  can  be  perceived  by  the  cells  of  the 
body  is  probably  some  form  of  motion,  the  movements  of  some  thing,  be  it 
a  solid,  a  fluid,  or  a  gas.  There  are  many  kinds  of  such  movements,  from 
the  slow  pressure  of  a  rigid  mass  up  to  the  rapid  motions  of  waves  of  the 
atmosphere  or  other  gases.  Also  there  are  certain  qualities  of  these 
movements,  such  as  direction,  duplication,  repetition,  and  rhythm  that 
can  be  perceived  by  some  cells  and  not  by  others. 

The  cells  which  can  only  perceive  the  movements  of  bodies  in  a  gen- 
eral way,  including  pressure  and  impact,  are  known  as  the  cells  0}  touch 
or  tactile  cells.  Those  which  make  use  of  the  impact  or  pressure  of  special 
bodies  under  the  influence  of  gravity  or  spatial  relations  to  determine 
the  position  of  the  body  are  the  cells  of  equilibration  or  static  cells.  Those 
which  perceive  the  motion  of  the  air  waves,  either  directly  or  as  trans- 
mitted and  represented  through  the  mechanical  vibrations  of  substances 
affected  by  these  waves,  are  called  the  sense  cells  of  hearing  or  the  auditory 
cells.  These  several  kinds  of  nervous  tissue,  all  alike,  perceive  only  the 
mechanical  movements  of  matter. 

It  is  characteristic  of  these  tissues  that  the  perceptory  cell  itself  almost 
never  receives  the  stimulus  first  hand,  but  usually  from  some  other  and 
intervening  tissue  cell  or  dead  cell,  cell-product  or  foreign  body  that, 
while  itself  not  sensitive,  is  yet  able  to  convey  the  motion  stimulus  to  the 
real  perceptory  cell  by  transmitting,  sometimes  with  modifications,  the 
movement  through  its  own  mass.  We  shall  call  such  bodies  the  inter- 
mediate tissues  or  substances.  These  intermediate  cells  and  materials 
form  organic  parts  of  the  sensory  apparatus,  many  of  which  would  not 
be  able  to  operate  without  them  or  would  send  in  exaggerated  tactile 
or  pain  sensations,  or  none  at  all. 

Some  few  forms  of  tactile  cells,  found  in  various  groups  of  animals, 
receive  the  stimulus  almost  directly  upon  their  sensory  surfaces  and 
practically  without  the  aid  of  intermediate  cells  or  tissues.  Such  a  case 


TACTILE    TISSUES 


201 


is  the  tactile  cell,  whose  afferent  process  terminates  in  a  nerve-ending 
in  the  stratified  epithelium  of  the  cornea  of  mammals.  It  was  formerly 
thought  that  the  branching  fibrils  of  this  ending  lay  free  upon  the  corneal 
surface.  They  probably  reach  almost  if  not  quite  to  it.  The  least  touch 
of  a  foreign  solid  practically  reaches  these  endings  directly  and  causes 
them  to  transmit  a  sensation  of  pain  to  the  nerve  centers.  They  come 
very  near,  if  not  entirely,  to  being  sensory  tactile  endings  that  operate 
directly  in  response  to  the  touch  of  solids  and  not  through  the  agency  of 
any  intermediate  cells  or  substance. 

This  form  is  little  removed  in  similarity  of  structure  and  operation 
from  the  great  mass  of  tactile  perceptory  organs  found,  in  the  majority  of 
animals,  at  the  periphery.  These  ,  ,._  .-__  _,_— — •  _.  -_^> 

organs  utilize  the  living  or  (less 
often)  dead  cells,  among  which 
they  branch,  as  intermediate  or- 
gans, through  which  they  receive 
their  motion  stimuli.  When  the 
intermediate  tissue  is  removed, 
by  accident  or  otherwise,  the  tac- 
tile sensation  is  intensified  to 
one  of  pain.  A  good  example 
of  a  large  and  widely  distributed 
group  is  to  be  seen  in  the  affer- 
ent sensory  endings  of  nerve  cells 
in  the  skin  on  the  snout  of  the 
pig  (Fig.  179).  This  figure  needs 
but  little  explanation,  showing 
the  end-organ  fibrils  passing  in  angular  paths  among  the  stratified 
epithelial  cells  to  within  two  or  three  cells  of  the  surface.  These 
epithelial  cells  are  not  sensory.  They  can  feel  nothing.  But  the  slight- 
est touch  on  the  surface  pushes  them  against  the  nerve-endings,  and 
the  sensation  is  carried  to  the  nerve  centers.  Nor  are  these  epithelial 
cells  specialized  to  do  their  work  in  any  way  that  can  be  detected.  Any 
other  cells  in  this  position  would  do  just  as  well.  In  fact,  many  of  them 
are  dead  cells. 

In  the  columnar  cells  of  many  animals  which  have  no  stratified  epi- 
thelium in  the  epidermis  we  find  a  similar  arrangement.  In  the  earth- 
worm such  a  sensory  structure  is  found  (Fig.  180).  Here  the  sensory 
fibrils  are  placed  around  and  between  the  proximal  ends  of  the  epithelial 
cells  which  convey  the  motion  stimulus  to  them  in  exactly  the  same  way 
that  the  stratified  cells  did  in  the  pig's  snout.  We  must  realize  the 
difficulty,  however,  of  distinguishing  these  sensory  endings  from  motor 
endings  used  to  stimulate  the  gland  cells  of  this  epithelium  into  secretion. 


FIG.  179.  —  Sensory  nerve  end-organ  in  the  exter- 
nal epithelium  of  a  pig's  snout.    (After  RZTZIUS.) 


2O2 


HISTOLOGY 


A  step  higher  in  the  relations  of  a  tactile 
perceptory  cell  to  its  intermediate  cells  is 
to  be  seen  in  the  epithelium  of  the  cat's 
foot  or  the  Guinea  pig's  skin  (Fig.  181). 
Here  the  sensory  ending  comes  into  contact, 
by  cup-shaped  swellings  called  menisci,  with 
certain  of  the  epithelial  cells  instead  of  with 
any  of  them.  Those  so  distinguished  are 
called,  erroneously  perhaps,  tactile  cells.  If 
they  can  feel  and  transmit  their  sensation 
as  an  impulse  to  the  meniscus-bearing  cells, 
then  they  would  be  true  tactile  cells,  and 
the  menisci  would  belong  to  communica- 
tory cells.  As  they  are  most  probably  only 
differentiated  slightly  to  intensify,  soften,  or 
otherwise  qualify  the  motion  which  they 
mechanically  transmit,  they  are  rather  to 
be  regarded  as  slightly  specialized  interme- 
diate cells  and  not  of  any  nervous  function 
whatever. 

We  shall  now  examine  a  group  of  tactile 
organs  with  highly  developed,  multicellular, 
intermediate  tissues.  These  intermediate 
tissues  are  constructed  so  as  to  transmit 


FIG.    180.  —  Tactile   nerve-endings 
in  the  integument  of  the  earth- 

TeUsT  £;  ^ptoTce^    the  pressure  to  the  nerve-ending  on  the  hy- 
gans  projecting  through  the  cuti-    draulic  principle  that  any  pressure,  exerted 

SCHNEIDER  after  R.     fr()m  ^  outside  upon  the  waUs  Q£  a  dosed 

cavity  containing  a  fluid,  will  be  transmitted 
to  all  inner  surfaces  of  the  walls  and  also  to  the  surfaces  of  any  objects 
contained  therein,  at  a  certain  ratio  per 
unit  of  surface. 

The  nerve-endings  from  one  or 
more  fibers,  in  these  organs,  enter  the 
interior  of  a  closed  sac  formed  by  sev- 
eral coverings  of  a  lamellar  connective 
tissue.  Here  they  lie  in  a  quantity  of 
a  fluid  or  semifluid  substance  which 
transmits  the  pressures  that  they  are 
intended  to  perceive.  We  shall  study 
two  examples  of  this  kind  of  struc- 
ture tO  become  acquainted  with  the  FIG.  i8i.—  Sensory  (tactile)  nerve-endings 
two  Chief  variations.  among  the  stratified  epithelial  cells  of  a 

•  cat's    toe.      X  600.      (After  DOGIEL    in 

The   first  of   this   kind  of   nerve-      Arch.f.mik.  Anat.) 


TACTILE    TISSUES 


203 


ending,  found  in  the  connective  tissue  lying  between  the  muscles 
of  the  cat,  is  used  apparently  to  record  the  pressures  produced  by 
the  movements  of  the  body.  Each  of  them,  known  as  a  cylin- 
drical corpuscle,  consists  of  a  very  plain,  al- 
most straight  and  somewhat  granular,  single 
nerve  termination,  the  end  of  a  nerve  fiber  to- 
gether with  other  tissues  placed  around  it.  Its 
extreme  end  is  irregularly  bent.  The  granular 
material  in  which  it  lies  is  called  the  inner  bulb 
and  is  probably  a  product  of  the  capsule  cells. 
The  connective-tissue  coverings  are  only  four  or 
five  in  number,  with  nuclei  occurring  at  fre- 
quent intervals  between  the  layers.  These  con- 
nective-tissue cells  form  plate-like  areas  of  the 
connective-tissue  substance  and  the  plates  are 
joined  into  a  series  of  coverings.  Consequently, 
the  appearance  of  the  coverings  is  the  same  in 
any  section  that  cuts  the  central  axis,  a  series 
of  thread-like  rings  of  tissue  with  the  nuclei 
scattered  between  them.  The  coverings  are  con- 
tinuous with  the  sheath  of  the  nerve  (Fig.  182). 

Various  modifications  of  this  simple  form  are 
found,  with  more  or  less  coverings  and  with  vari- 
ations in  the  shape  of  the  nerve-ending,  which, 
however,  is  primarily  a  single,  approximately  straight  rod.  Its  modi- 
fications consist  of  granulations  and  fine  side  processes.  Sometimes 
a  second  small  fiber  enters  the  capsule  and  forms  a  plexus  around  the 
main  termination. 

The  second  specimen  of  a  simple  encapsulated  nerve  end-organ,  whose 
intermediate  structures  operate  on  the  hydraulic  pressure  plan,  is  found 
in  the  skin  and  other  parts  of  the  periphery 
of  mammals.    The  differences  which  it  and 
the  rest  of  its  class"  exhibit,  when  compared 
with  those  just  mentioned,  are  a  branching 
and  anastomosing  end-organ  whose  fibrils 
are  irregular  in  course  and  in  shape,  but 
whose  contour  is  smooth  instead  of  granular, 
Methyiene  blue'  picture.    (After  as   was   the    case  in  the   preceding   form. 

DOGIEL  in  Arch.f.  mik.  Anat.)        ^^   .^  ^  &  somewhat  Dinner  Capsule, 

which  probably  permits  other  sorts  of  motions  and   pressures   to  be 
transmitted  than  the  purely  hydraulic  kind. 

A  type  of  this  form  is  shown  in  the  end-bulb  of  Krause  from  the 
conjunctiva  of  man  (Fig.  183).    The  various  other  forms  of  this  group, 


septum  of  cat,  showing  the 
outer  capsule  and  the  in- 
ner granular  cytoplasm 
which  contains  the  rod- 
like  nerve-ending.  (From 
BOHM  and  DAVIDOFF'S 
"  Histology  "  by  HUBER.) 


204 


HISTOLOGY 


as  the  Meisner's  corpuscles  and  the  genital  corpuscles,  etc.,  differ  from 
this  specimen  merely  in  general  shape,  size,  and  the  pattern  into  which 
the  terminal  end  fibrils  are  woven. 

In  contrast  to  these  two  forms  of  touch  organs  in  which  a  non-cellular 
substance,  a  granular  fluid,  acts  hydraulically  upon  the  nerve-ending, 
we  must  note  three  other  kinds,  all  of  which  have  capsules,  but  in  which 
the  chief  content,  which  is  in  direct  contact  with  the  nerve-ending,  is  a 
solid  cellular  mass.  These  are  the  corpuscles  of  Herbst,  the  neuromus- 
cular  and  the  neurotendinous  tactile  organs. 

In  the  corpuscles  of  Herbst  and  some  others  that  resemble  it  in  struc- 
ture, the  nerve  enters  the  melon-shaped,  many-layered  capsule  (which  is 

almost  exactly  like  that  of  a  Pacinian 
corpuscle)  and  ends  in  a  rod  much  as 
in  the  cylindrical  corpuscle.  Instead 
of  being  surrounded  by  a  non-cellular, 
fluid  product  of  cells,  however,  it  lies 
in  a  single  layer  of  heavy,  cubical  cells 
which  form  a  cylindrical  covering 
around  it.  These  cells  are  thus  placed 
between  the  nerve-ending  and  the  in- 
nermost covering  of  the  capsule.  We 
can  find  this  organ,  to  study,  in  the 
skin  on  the  bill  of  the  duck  and  other 
waterfowl  (Fig.  184).  The  inner  cu- 
bical cells  are  of  unknown  function, 
but  &  is  very  probable  that  they  act 

ending  in  the  integument  of  a  duck's  bill,    as     modifiers    of     the     numerous    jars, 

^SSFJZSSSS.'Z.  "*•.  tou*es.  «<•'  *>  which  «*  duck 

rounding  the  reticular  end-organs  of  the    Subjects    its    Sensitive  bill    in    Order    to 

learn  the  nature  of  objects  by  the 
delicate  sense  of  touch. 
In  the  muscles  and  tendons  of  some  vertebrates  are  found  sensory 
organs  composed  of  spindle-shaped  areas  of  the  muscle  or  tendon  tissue 
itself,  inclosed  in  a  capsule  and  provided  with  the  afferent  fibers  of  sen- 
sory touch  cells  that  enter  the  capsule  and  branch  freely  on  the  inclosed 
and  slightly  modified  muscle  fibers  or  tendon  fibers  within.  These 
organs  record  the  pressures  to  which  the  muscles  and  tendons  are  sub- 
jected during  the  contraction  of  the  muscles,  etc.  If  the  pressure  be- 
comes too  great,  the  sensation  becomes  one  of  pain.  These  are  called 
the  neuromuscular  and  the  neurotendinous  organs  of  touch.  In  both  of 
them  the  method  of  distribution  of  the  terminal  organs  is  essentially 
the  same,  but  also  very  different  from  the  pattern  formed  by  these  organs 
in  all  the  other  touch  structures.  The  fiber  branches  into  several  fibrils 


nerve    fiber. 
A.  DOGIEL.) 


.-  Tactile  (and  taste?)  nerve- 


(From    Anat.   Anz.   after 


TACTILE    TISSUES 


205 


which  end  in  a  number  of  telodendria  on  the  surfaces  of  the  rudimentary 
muscle  and  tendon  fibers  (Fig.  185). 


nv.end, 


FIG.  185.  —  Neurotendinous  nerve  end-organ  in  the  rabbit.     (From  HUBER  and  DE  WITT  in 
Journ.  of  Comp.  Neurology.) 

All  the  above  organs  were  found  placed  in  the  soft  tissues  of  animals, 
and  no  hard  parts  were  developed  in  connec- 
tion with  them.  It  is  true  that  the  cuticle 
of  the  earthworm  intervened  between  the 
source  of  the  motion  and  the  simple  epithe- 
lium cells  that  more  directly  acted  to  pass  the 
movement  on  to  the  nerve-ending,  but  it  can- 
not be  said  that  this  cuticle  was  developed 
in  any  way  to  perform  this  as  a  duty.  The  two 
structures  to  be  demonstrated  now  will  each 
show  such  a  rigid  organ,  in  the  one  case  a 
cuticle  which  is  a  cell-product,  and  in  the 
other  a  rigid  hair  made  from  the  dead  and 
hardened  bodies  of  the  cells  themselves, 
which  is  developed  solely  to  act  as  an  inter- 
mediate structure  in  transmitting  the  motion 
stimulus.  These  are  the  tactile  hairs  of  the 
crustacean,  Palamonetes,  and  the  tactile  hairs 
or  vibrissae  of  the  cat  and  other  mammals. 

The  crustacean  in  question  has  much  the 
same  sort  of  integument  that  the  earthworm 
had,  —  a  layer  of  simple  columnar  epithelial 
cells  that  produce  a  cuticle.  One  of  the  chief 

differences  is  that  the  cuticle  is  thicker  and,  FlG.  l86._  Tactile  end-organ  of 
in  addition,  is  usually  impregnated  with  salts 
of  lime.     This  makes  it  much  harder  and 
more  resistant  to  the  perception  of  outside 
movements  through  its  boundaries.  The  sen- 
sory nerve  supply  comes  to  the  periphery  as 
it  did  in  the  earthworm,  and  could  receive 
the  tactile  stimuli  much  as  the  earthworm's  nerve-endings  did  were  it 
not  for  the  thick  and  hard  shell.     A  simple  modification  of  the  whole 


a  nerve  fiber  in  the  tactile  hair 
of  a  shrimp,  Palamonetes.  cu., 
cuticle;  hyp.,  hypodermis,  which 
is  invaginated  at  (h.c.)  into  the 
hair  cells,  h.,  outer  structure  of 
the  hair;  nv.end.,  nerve-ending 
in  the  lower  part  of  the  hair. 
(After  PRENTISS.) 


206 


HISTOLOGY 


neighboring  region  takes  place,  where  each  tactile  ending  comes 
to  the  periphery,  so  that  the  slightest  stimulus  can  be  received. 
The  simple  epithelium  at  such  a  point  is  invaginated  into  a  group 
of  hair  cells  (Fig.  186,  h.c.)t  whose  distal  ends  are  lengthened  out 
so  as  to  make  a  long,  thin,  hairlike  projection  (h.)  from  the  surface 
of  the  body.  These  cells  form  a  cuticle,  as  do  all  other  epithelial 
cells,  but  it  is  not  as  thick  as  that  on  the  rest  of  the  body,  and  on 
account  of  the  form  of  the  cells  from  which  it  is  derived  it  is  shaped 
like  a  hair.  The  nerve-ending  is  single  and  large.  It  probably  rep- 
resents the  ending  of  a  single  cell,  and  not  one  of  the  numerous 
branches,  as  do  the  endings  of  like  nature  in  the  pig's  snout.  It  lies 
in  a  position  homologous  to  that  occupied  by  the  tactile  endings  in 

the  other  epithelia  at  the  bases 
of  the  epithelial  cells.  It  ex- 
tends up  into  the  fiber  in  this 
position  for  a  considerable  dis- 
tance, and  the  latter  part  of 
its  course  must  represent  a 
passage  between  the  sides  of 
the  cells,  while  the  earlier  part 
was  undoubtedly  a  contact  with 
their  bases. 

The  second  example  is  that 
of  the  mammal's  tactile  hair. 
For  the  structure  of  the  hair 
itself,  see  the  part  devoted  to 
this  subject  in  Chapter  XX. 
The  nerve  supply  consists  of  the 
terminal  branches  of  a  single 

ctile,   sensory   nerve-endings   placed    fiber  which  approaches  the  hair 


n.f. 


nv.  end. — 


FIG.  187. — Ta 


work  end-organs  at  nv.end.     (From  EDINGER  and    branches    that    encircle    it    just 
HALL,  after  VAN  GEHUCHTEN.)  bdow      the       mouths      of       fog 

sebaceous  glands.  This  ring  gives  off  branches  that  are  naked  and  vari- 
cose and  run  a  short  distance  distally  before  terminating.  There  are 
a  considerable  number  of  these  end  fibrils  which  lie  outside  the  glassy 
layer  of  the  follicle  (Fig.  187). 

The  impulse  in  this  case  is  transmitted  in  almost  exactly  the  same 
way  as  in  the  crustacean's  tactile  hair,  although  the  two  hairs  are  so 
differently  formed.  In  both  the  hair  acts  as  a  lever,  transmitting  the 
slightest  contacts  with  its  outer  portions,  as  greatly  intensified  motion 
stimuli  to  the  nerve  end-organs  at  its  base.  In  one  case  the  nerve  ter- 
mination is  inside  the  hair,  while  in  the  other  it  is  outside.  Some  mol- 


STATIC   TISSUES  2O; 

lusks,  as  Chiton,  have  analogous  organs  of  touch  on  their  shell-covered 
surfaces. 

One  other  perceptory  power  must  be  considered  here,  and  that  is  the 
perception  of  heat  and  cold.  Certain  parts  of  the  body  surface  in  man 
perceive  very  small  differences  of  temperature,  while  others  can  only  feel 
much  greater  changes.  When  great  extremes  are  properly  applied,  they 
give  the  same  result,  a  sense  of  intense  cold,  which  means  a  destruction 
of  the  organ  and  consequently  of  the  power  to  perceive  any  heat. 

These  sensory  endings  have  not  been  discovered  histologically.  It 
is  possibly  true  that  they  are  some  of  the  same  end-organs  that  also 
perceive  contact  or  other  tactile  stimuli.  Or  they  may  be  specialized 
to  perform  the  thermo-perceptory  function  alone.  It  might  prove  pos- 
sible to  discover  them  by  a  process  of  comparison  and  elimination  in 
the  various  regions  that  possess  them  or  do  not  possess  them. 

Technic.  — The  ordinary  sectioning  and  staining  methods  are  of  no 
value  in  the  study  of  these  tissues.  Silver  nitrate  gives  some  good  pictures 
of  the  structure,  but  the  methylene-blue  method  is  the  principal  means 
by  which  we  have  attained  our  present  knowledge  of  their  structure. 
This  method,  we  must  repeat,  is  not  one  that  cannot  be  learned  out  of  a 
book.  The  instructor  must  help  and  advise  the  student,  and  they  must 
adapt  one  of  the  numerous  forms  of  this  method,  as  set  out  in  Lee,  to 
their  needs. 

LITERATURE 

The  literature  is  very  large.     Good  papers  may  be  found  as  follows :  — 
RETZIUS,  G.     Papers  in  Biol.  Untersuch.,  Jena.     Last  ten  years.     See  Vol.  1902. 
DOGIEL,  A.  S.     Articles  in  the  Arch,  fur  mik.  Anal.,  Band  XLIV,  S.  15,  Band  XXXVII, 

S.  602,  Band  XLIX,  S.  769,  Band  LII,  S.  44,  Band  LIX,  S.  i. 
HUBER,  G.  C.     " Neuro-muscular  Spindles  in  the  Intercostal  Muscles  of  the  Cat,"  Am. 

Journ.  of  Anat.,  1902,  p.  i. 
PRENTISS,  C.  W.     "The  Otocyst  of  Decapod  Crustacea,"  Bull.  Museum  of  Comp.  Zodl., 

Harvard,  Vol.  XXXVI,  i9oi. 

THE   TISSUES    OF    EQUILIBRATION    OR    STATIC   TISSUES 

The  static  tissues  or  tissues  of  equilibration  are  tissues  that  record 
the  position  of  the  body  or  some  larger  part  of  it  with  regard  to  gravity 
or  to  the  body's  successive  positions  in  space.  It  is  impossible,  some- 
times, to  differentiate  between  these  two  forms  of  the  function;  while 
in  other  examples,  very  good  evidence  has  been  obtained  to  separate 
them.  These  tissues  appear  as  an  epithelium. 

As  has  been  said,  this  function  must  be  looked  upon  as  a  very  delicate 
form  of  touch  whose  tissues  are  so  placed  as  to  perceive  and  record  the 
positions  of  small  and  heavy  bodies  which  press  or  strike  against  them 
by  gravity,  or,  by  inertia  when  the  organism  moves  and  changes  its 
position.  The  flow  of  fluids  over  the  sensitive  surface  of  some  of  the 


208 


HISTOLOGY 


static  cells  also  tells  the  animal  when  its  body  has  moved  or  turned  in 
certain  directions. 

With  a  few  exceptions  (the  Crustacea),  and  on  account  of  the  great 
delicacy  of  the  stimulus,  the  static  cells,  unlike  the  coarser  tactile  cells, 
receive  the  impression  directly  upon  their  perceptory  end-organ.  This 
would  mean  that  they  possess  no  intermediate  tissues  unless  we  may 
consider  the  crystals  of  lime,  chitin,  and  foreign  matter,  that  are  operated 
on  them  by  gravity  or  inertia,  as  intermediate  tissues.  Since  these 
carbonate  of  lime  concretions,  chitinous  plates,  and  even  foreign  bodies, 
as  grains  of  sand  or  streams  of  fluid  are  necessary,  and  often  organic, 
parts  of  the  apparatus,  we  shall  designate  them  as  the  intermediate  sub- 
stances. 

On  account  of  the  delicate  nature  of  the  tissues  and  their  operations, 
the  surface  that  contains  them  is  usually  invaginated  into  some  sac  or 
follicle  inside  the  body.  This  leaves  a  mouth  or  duct  leading  to  the 
exterior  and  remaining  open  in  primitive  forms  of  the  organ,  but  closed 
and  cut  off  entirely  in  the  more  specialized  forms. 

The  perceptory  end-organs  of  the  static  nerve  cell,  which  are  rod-like 
or  hair-like  processes,  are  placed  directly  upon  the  cell  body,  which  is  a 
part  of  the  surface  of  the  static  epithelium.  The  only  exception  to  this 
is  the  same  case  mentioned  above  as  an  exception,  and  it  will  well  serve 

as  our  first  subject  of  study,  the  static 
organs  of  the  Crustacea. 

These  structures  will  show  us  bet- 
ter than  any  others  the  relation  of  the 
static  organs  to  the  tactile  organs.  In 
the  prawn,  Palamonetes,  the  static 
organ  consists  of  an  invaginated  hol- 
low or  chamber  in  the  side  of  a  joint 
of  the  claw.  This  chamber  remains 
in  communication  with  the  exterior  by 
means  of  its  opening  at  the  point  of 
invagination. 

Inside  of  this  pocket  we  find  some 
particularly  delicate  "  touch  hairs." 
That  is  to  say,  they  are  formed  like 
touch  hairs  in  general,  except  that 
they  have  finer  and  more  numerous 
ends  and  that  they  are  bent  so  as  to 
have  their  sides  most  accessible  to  any- 
thing that  may  touch  it  (Fig.  188). 

These  hairs  are  situated  in  groups  on  various  parts  of  the  interior  surface 
of  the  sac  and  are  played  upon  by  grains  of  sand,  etc.,  which  the  shrimp 


fi 


FIG.  188.  —  A  static  sensory  hair  from  the 
statocyst  of  the  shrimp,  Palamoneles. 
Formed  similarly  to  the  tactile  hair  (Fig. 
1 86)  but  with  a  thinner  cuticle  on  the 
hair;  CM.,  cuticle;  hy.c.,  hypodermal 
cells;  h.c.,  hair  cells;  nv.fi.,  nerve  fiber. 
(After  PRENTISS.) 


STATIC   TISSUES  2OQ 

manages  to  insert  in  the  sac  for  that  purpose.  The  animal  is  conscious, 
when  a  hair  is  touched  by  the  sand  grain,  of  which  part  of  the  sac  the 
hair  was  in  and  is  able 

to  guide  its  movements  v  J^jf|  |  ^j^,'  stl 

accordingly.  By  "  con- 
scious "  is  not  meant  the 
same  thing  that  the 
word  would  imply  in  a 
mammal,  but  a  correla- 
tion of  the  particular 
hairs  touched  with  the 
use  of  certain  muscles 
to  maintain  the  animal 

in     an     Upright     position  FlG-  l89-—  Tentaculocyst  (statocyst)  of  a  medusa,  Rhopa- 

.                                             .  lonema.    stl.,  statolith  inclosed  in   a  pedicle   which  sways 

With    regard    tO    gravity.  Witn  the  animal's  motion  and  records  its  movements  by  the 

Compare  Figure  1 88  with  hairs  that   Project  from   its  surface.     (From   LANG  after 

,        ,         •           /•     i  HERTWIG.) 

the  drawing  of  the  tac- 
tile hair  shown  in  Figure  186  and  see  how  essentially  alike  the  two  are. 

Whether  the  action  of  the  sand  particles  or  statoliths  upon  the  static 
hairs,  in  this  creature,  will  convey  to  the  nerve  centers  an  accurate 
measure  of  spatial  movement  or  not  is  not  known. 

An  example  of  an  organ  probably  used  to  determine  the  direction  of 
movements  of  the  body  in  space  is  to  be  seen  in  some  of  the  static  organs 
of  medusae.  In  these  forms  a  body  surface  is  invaginated  into  a  more 
or  less  complete  sac,  and  from  the  lower  wall  of  this  sac  a  pedicle  arises, 
containing  in  its  tissue  a  crystal  or  concretion  of  lime,  or  sometimes 
several  of  them  (Fig.  189). 

The  cells  lining  the  walls  of  both  the  cavity  and  the  pedicle,  which 
is  called  a  tentaculocyst,  are  provided  with  sensory  hairs,  and  the  least 
motion  of  the  body  must  convey  a  record  of  action  by  the  touching  of  the 
sensory  hairs  on  one  side  or  the  other  of  the  tentaculocyst  and  cavity. 
While  gravity  probably  makes  some  record  of  its  pull,  the  waving  of 
the  body  edge  to  and  fro  in  swimming  must  cause  a  much  greater  stimu- 
lus, and  the  organ  therefore  records  the  motions  of  the  body  in  space 
by  the  inertia  of  its  heavy  statoliths. 

Auditory  cells  have  been  described  as  formed  on  the  walls  of  these 
cysts.  While  admitting  the  possibility  of  this,  the  writers  do  not  think 
that  the  cells  described  can  be  used  to  hear  sound. 

A  beautiful  example  of  a  statocyst,  used  to  orient  a  part  of  the  body, 
is  to  be  seen  in  the  organ  that  occurs  in  the  "foot  "  of  some  plecypod  mol- 
lusks.  Figure  190  shows  a  picture  drawn  from  a  section  of  the  statocyst 
in  a  small  plecypod,  Cyclas,  a  fresh-water  form.  The  cavity  of  this  cyst, 
which  was  invaginated  and  cut  off  from  the  ectoderm,  is  lined  with  an 


2IO  HISTOLOGY 

epithelium  that  also  consists  of  two  kinds  of  cells.  The  sensory  cells, 
bearing  each  from  60  to  100  long,  stiff  sensory  hairs,  lie  apart  and  touch 
each  other  only  by  the  edges  of  their  flange-like  upper  surfaces.  Be- 
tween them,  and  wedged  up  almost  in  a  line  with  them,  lie  the  large 
supporting  cells  which  seem  to  be,  in  this  case,  of  connective-tissue  origin 
on  account  of  their  not  touching  the  epithelial  surface.  An  embryologi- 
cal  study  of  these  cells  would  determine  their  origin.  Lying  in  the 
cavity,  and  just  clear  of  the  long  sensory  hairs,  is  the  round  statolith  which 
has  an  organic  basis,  probably  of  chitin,  that  is  impregnated  with  car- 


FIG.  190.  —  Statocyst  of  a  species  of  Cyclas.    stl.,  statolith;  sen.c.,  sensory  cells;  sup.c.,  sup- 
porting cells ;  sen.r.,  sensory  rods. 

bonate  of  lime.  The  pressure  and  impact  of  this  statolith  on  one  or  the 
other  of  the  groups  of  sensory  hairs  must  tell  exactly  in  what  position 
the  foot  is  lying,  and,  consequently,  which  way  it  is  to  be  moved  next. 
The  foot  moves  entirely  independently  of  the  position  of  the  rest  of  the 
body.  Watch  a  Unio,  or  better,  an  Ensatella,  that  has  been  dug  up 
and  left  on  the  wet  sand.  Turn  it  in  different  positions  and  see  how  its 
foot  always  attempts  to  go  down. 

This  organ  should  be  studied  in  the  living  embryos  and  young  of 
Cyclas.  Here  it  is  most  easily  observed,  and  the  statocyst  is  seen  to  be 
in  constant,  gentle  motion.  We  can  therefore  conclude  that  it  stimulates 
the  hairs  by  a  rhythmic  impact  rather  than  by  a  pressure  or  single  impact. 

Various  forms  of  this  same  organ  occur  through  the  mollusk  series, 


STATIC   TISSUES 


211 


bos. 


the  highest  development  being  reached,  probably,  in  the  statocysts  of 
the  active  cephalopods  where  the  two  organs  are  much  enlarged  and  the 
epithelium  is  much  differentiated.  It  is  possible  that  part  of  it  has  an 
auditory  function  in  the  squid.  This  animal  has  its  two  "  otocysts  " 
(we  shall  hereafter  term  them  "  statocysts  ")  enlarged  into  spaces  of 
some  size  and  considerable  differentiation  as  to  shape.  Several  bars 
of  the  surrounding  capsule  of  cartilage  project  into  this  space ;  and  the 
epithelium  which  lines  it,  while  it  is 
thin  and  undifferentiated  in  most 
regions,  has  several  strongly  special- 
ized portions.  The  use  of  these 
highly  differentiated  portions  can  be 
partly  inferred  from  the  habits  of 
the  animal.  No  creature  has  better 
control  of  its  swift  movements  and 
rapid  changes  of  position,  whether 
swimming  in  schools  at  sea  or  in 
following  the  twisting  and  turning 
of  its  agile  prey,  the  mackerel,  and 
other  fish.  It  is  probable,  therefore, 
that  the  best-developed  sensory  epi- 
thelium of  this  statocyst  is  used  to 
record  and  so  control  its  motion.  It 
is  improbable,  although  possible, 
that  the  statocyst  has  some  auditory 
function  to  perform. 

The  most  specialized  of  the  lining 
epithelium  is  found  on  the  median 
posterior  side  of  the  sac.  Here  the 
cells  have  formed  several  layers,  the 
most  distal  of  which  lie  in  several  rows  and  are  provided  with  nu- 
merous rods  which  resemble  cilia  in  appearance.  As  all  other  classes 
of  mollusks  have  moving  cilia  in  this  position,  it  is  possible  that  these 
rods  move  and  are  cilia,  or  at  least  are  modified  cilia.  The  intra-cellular 
portions  of  these  rods  are  furnished,  inside  the  cell,  with  several  rows 
of  spindle-shaped  enlargements  or  knobs  known  as  blephroplasts  (Fig. 
191).  The  sensory  cells  lie  among  more  numerous  sustentacular  cells. 

A  layer  of  cells  containing  ganglion  cells  lies  beneath  these  perceptory 
cells  and  separates  them  from  the  underlying  capsule  of  cartilage. 

The  insects  undoubtedly  have  great  static  powers.  — Their  life  and 
actions  show  this.  And  yet  no  great  specialization  of  any  tissue  for  this 
purpose  has  been  found.  The  only  case  in  which  such  an  organ  has 
been  surmised  is  in  the  Diptera  or  flies.  These  animals  have  the  rudi- 


FIG.  191.  —  Static  sensory  cell  from  the  stat- 
ocyst of  a  squid,  Loligo  Pealii.  nu.,  nu- 
cleus; sen.r.,  sensory  rods;  bas.gr.,  basal 
granules  of  two  grades.  (From  a  draw- 
ing by  A.  F.  McCiJNTOCK  and  E.  W. 
BIXBY.) 


212 


HISTOLOGY 


ments  of  the  second  pair  of  wings  developed  into  the  so-called  "  balancers." 
The  writers  have  found  by  experiment,  however,  that  flies  can  fly  as 
well  without  these  organs  as. with  them.  Insects  certainly  cannot  use 
sight  to  maintain  their  static  equilibrium.  Most  of  them  cannot  see  far 
enough  or  well  enough  to  do  this.  And  even  when  blinded  or  in  dark- 
ness, the  static  sense  is  not  gone.  Their  activities  would  also  demand 
a  spatial  sense,  and  yet  evidences  of  structures  used  for  this  purpose  have 
not  been  found. 

The  static  tissues  of  the  vertebrates  have  been  complicated  by  the 
specialization  of  parts  of  their  static  epithelia  to  hear  sound,  as  is  de- 


8t.C. 


FIG.  192.  —  Part  of  a  longitudinal  section  of  an  ampulla  from  the  skate,  Raja  lavis,  showing  a 
transverse  section  of  the  sensory  epithelium  on  the  medium  septum,  gen.ep.,  general  epi- 
thelium composed  of  static  cells  (st.c.)  and  supporting  cells,  sen.  ep.,  sensory  epithelium. 
A  nerve  fiber  can  be  seen  entering  the  epithelium  and  dividing. 

scribed  in  another  part  of  this  chapter.  We  shall  study  the  sensory 
epithelium  found  in  the  ampullae  of  the  semicircular  canals  as  probable 
examples  of  tissues  which  perceive  the  spatial  movements  of  the  body, 
or  of  the  head  when  that  part  is  moved  independently.  The  origin  and 
general  relations  of  the  semicircular  canals  have  been  indicated  in 
another  part.  We  may  repeat  here  that  they  are  integral  parts  of  the 
statocyst  cavity  and  that  one  end  of  each,  where  it  joins  the  utriculus, 
is  enlarged  in  diameter  to  form  an  ampulla  of  which  there  are  three,  one 
for  each  canal.  This  ampulla  contains  an  oval  cavity,  in  the  fish  which 
our  figure  represents,  and  the  lining  epithelium  covers  the  entire  interior 
of  this  cavity,  including  a  ridge  rising  across  its  middle  at  right  angles 
to  the  length  of  the  lumen.  This  ridge  rises  almost  exactly  halfway  up, 


STATIC   TISSUES 


213 


sen.c.— 


thus  cutting  the  lumen  down  to  one  half  its  diameter  at  that  point,  which 
is  the  widest  point  in  the  ampulla,  and  consequently  in  the  whole  length 
of  the  tube. 

The  sensory  cells  are  placed  on  the  edge  of  this  ridge,  a  transverse 
section  of  which,  from  Raja  lavis,  is  represented  in  Figure  192,  also 
an  enlarged  figure  of  the  same  , 

structure  in  Amieurus  by  Figure 
193.  These  cells  are  rather  large, 
and  do  not  reach  down  to  the 
basement  membrane,  being  sup- 
ported by  contact  with  support- 
ing or  sustentacular  cells  whose 
proximal  ends  rest  by  broadened 
bases  on  a  well-developed  mem- 
brane. 

The  sensory  cells  have,  as  cell- 
organs  of  perception,  peculiar  fila- 
ments projecting  from  their  distal 
surfaces.  These  filaments  (Fig. 
193)  are  grouped  into  a  single 
projection  which  is  very  delicate 
and  is  partly  embedded  in  a  ge- 
latinous coating  which  covers  the 
ridge.  The  nuclei  are  placed 
about  halfway  in  the  height  of 
the  cell  and  are  oval,  with  a  dis- 
tinctive chromatin  pattern.  They 
are  larger  and  clearer  than  the 
nuclei  of  the  sustentacular  cells. 
These  latter  cells  have  the  nucleus 
a  little  lower  than  the  middle  and  entirely  below  the  row  of  static  nuclei. 

Medullated  nerve  fibers  enter  the  connective  tissue  of  the  ridge  freely 
and  extend  up  in  all  directions  to  the  epithelium.  Passing  through  the 
basement  membrane,  they  usually  lose  their  medullary  sheath  and  divide 
into  smaller  fibril-bundles  or  into  single  fibrils  to  innervate  the  sensory 
cells,  receiving  from  them  an  impulse  when  they  are  stimulated. 

This  stimulation  of  the  perceptory  cells  is  probably  performed  by 
currents  of  the  fluid  which  fills  all  parts  of  the  statocyst,  including  the 
canals  and  their  ampullae.  When  the  body  turns  in  any  direction,  the 
fluids  in  such  of  the  three  canals  as  lie  in  or  at  a  sharp  angle  to  the  plane 
of  motion  pass  backward  or  forward  through  the  canal  by  their  inertia, 
and,  flowing  over  the  ridge  so  well  placed  in  their  path,  stimulate  the 
sensitive  cells  on  its  edge  by  causing  their  delicate  processes  to  bend  and 


sup. 


FIG.  193.— Sensory  epithelium  from  median 
septum  of  ampulla  of  catfish,  Amieuris  catus. 
sen.c.,  sensory  cells;  sen.r.,  sensory  rods; 
supx.,  supporting  cells,  one  of  which  shows 
mitcsis;  b.m.,  basement  membrane  through 
which  two  nerve  fibers  pass.  One  fiber  is 
naked  while  the  other  carries  its  medullary 
sheath  into  the  epithelium.  They  both  ramify 
as  nerve-endings  on  the  bases  of  the  sensory 
cells. 


214 


HISTOLOGY 


nv.f: 


vibrate  in  the  current.    This  fluid  thus  acts  as  the  intermediate  sub- 
stance. 

It  is  possible  that  the  purely  static  function  with  reference  to  gravity 

is  performed  by  other  sensory 

jLJl__4     I  _^L— -Jw-— -tr""    areas  m  tne  larger  cavities  of 

the  sacculus.  Here,  particles 
of  lime  act  as  the  intermedi- 
ate substance  instead  of  a 
fluid,  as  in  the  ampulla,  or  a 
cuticular  structure,  as  in  the 
cochlea.  Figure  194  repre- 
sents a  nitrate  of  silver  prep- 
aration from  the  mouse  to 
show  the  nerve  distribution  in 
FIG.  i94.— Portion  of  macula  acustica  sacculi  of  such  a  sensory  region  of  the 

mouse,  treated  by  Golgi's  method  to  show  nerve-  ventrfculus.        This     region     IS 

ending  on   sensory  cells  (sen.c.*).     b.m.,  basement  ,  , 

membrane;  sup.nu.,  nuclei  supporting  cells;  nv.f.,  known    as  a  MOCUlar   dCUStlCd, 

nerve  fiber.     (After  VON  LENHOSSEK.)  and   its   Cells   greatly  resemble 

those  of  the  ampulla  ridge. 

Technic.  — To  have  a  complete  understanding  of  a  static  organ,  it  is 
necessary  to  have  a  great  variety  of  preparations.  The  finer  anatomical 
relations  must  be  studied,  and  these  can  best  be  got,  in  most  cases,  by 
well-prepared  serial  sections.  Such  series  are  difficult  to  prepare  on 
account  of  the  heterogeneous  tissues  that  go  to  form  these  organs.  In 
the  cases  of  the  higher  animals,  various  bones,  cartilages,  and  otoliths 
have  to  be  dealt  with  as  well  as  many  grades  of  connective  tissues.  A 
decalcifying  fixative  should  be  used  in  these  cases,  and  care  should  be 
taken  not  to  render  the  remaining  connective  tissues  unduly  hard  and 
brittle.  Zenker's  fluid  and  chrom-aceto-formal  were  very  successfully 
used,  sometimes  followed  by  a  double  embedding  in  paraffin  and  cel- 
loidin.  When  it  was  desired  to  stain  before  sectioning,  a  saturated  solu- 
tion of  sublimate  with  5  per  cent  of  acetic  acid  was  used  to  fix.  Silver 
and  methylene  blue  are  essential  in  making  a  study  of  the  nerve  elements. 
The  writers  also  found  that  carefully  teased  specimens  that  had  been 
macerated  somewhat  were  invaluable  in  class  demonstrations.  This 
latter  method  was  modified  as  follows  to  form  a  valuable  process  in  the 
study  of  any  epithelia.  The  tissue  was  first  placed  in  a  macerating 
medium  that  was  at  the  same  time  a  fairly  good  fixative;  weak  osmic 
and  chromic  acids,  as  ordinarily  used  to  macerate,  were  found  to  be  the 
best,  and  one  third  alcohol  also  gave  good  results.  The  tissue  was 
handled  with  the  greatest  caution,  and  somewhat  before  it  was  macerated 
enough  to  tease,  it  was  washed,  stained,  dehydrated,  and  embedded  in 
paraffin.  Sections  of  rather  greater  thickness  than  usual  were  then 


AUDITORY   TISSUES  21$ 

cut  and  laid  on  a  slide,  and  xylol  was  used  to  remove  the  paraffin.  The 
dropping  of  a  small  amount  of  balsam  was  now  enough  to  slightly 
separate  the  cells,  and  this  was  further  brought  about  by  the  placing  of 
the  cover  glass  in  position.  If  the  displacement  of  the  cells  was  too 
great,  a  little  collodion  and  clove-oil  mixture  was  first  placed  on  the  glass, 
and  this  served  to  retard  the  separation,  which  could  also  be  controlled 
in  many  other  ways.  The  result  is  perfect  in  all  ways  but  one ;  i.e.  the 
tissue  having  been  cut  in  sections  makes  it  impossible  to  say  if  each  cell 
in  the  finished  specimen  is  whole  or  not. 

LITERATURE 

OWSJANNIKOW  UNO  KowALEVSKY.  "  Uber  das  Centralorgan  und  das  Gehororgan  der 
Cephalopoden,"  Mem.  d.  I'Acad.  de  St.  Petersburgh,  T.  XI. 

MORRILL,  A.  D.  "The  Innervation  of  the  Auditory  Epithelium  of  Mustellus  canis," 
Journ.  of  Morph.,  Vol.  XIV,  p.  6,  pis.  VII  and  VIII. 

PRENTISS,  C.  W.  "The  Otocyst  of  Decapod  Crustacea,"  Bull.  Mus.  Comp.  Zool.,  Har- 
vard, Vol.  XXXVI. 


THE  TISSUES   OF   HEARING   OR   AUDITORY   TISSUES 

These  tissues  are  also  modified  forms  of  tactile  tissues.  They  are 
more  delicate  refinements,  even,  than  the  static  tissues,  since  they  per- 
ceive and  record  the  finest  differences  in  the  rhythm  of  successive  impacts 
caused  by  the  waves  of  the  atmosphere  that  are  known  as  sound. 

The  intermediate  tissues  in  this  case  consist  of  two  kinds.  One 
kind  is  intended  to  collect  and  thus  intensify  the  sound  and  to  convey 
it  to  the  auditory  tissues  which  are  usually  very  internal.  These  are 
known  as  tympana  and  pinnae.  A  second  kind  are  the  delicate  inter- 
mediate structures  that  directly  apply  the  sound  waves  to  the  auditory 
nerve  cells.  These  latter  must  be  of  exactly  the  right  texture  to  properly 
operate  upon  the  delicate  and  highly  specialized  perceptory  nerve-end- 
ings, which  here  consist  of  various  rods,  hairs,  or  plates.  In  some  forms, 
these  intermediate  substances  do  not  occur.  Where  they  do  occur,  they 
are  special  cell-products. 

In  the  vertebrates  the  auditory  tissues  are  specializations  of  the 
static  tissues,  and  therefore  the  two  are  found  to  be  closely  related  and 
parts  of  the  same  organ.  This  is  not  the  case  in  some  of  the  insects 
where  there  is  a  separate  origin  and  position. 

For  us  to  determine  whether  a  given  sense  organ  is  auditory  or  static 
in  function  is  sometimes  a  difficult  task,  when  we  investigate  such  organs 
as  are  other  than  our  own.  We  must  consider  various  points  in  this 
connection.  A  comparison  of  the  tissues  of  the  mammals  with  our  own, 
structurally,  tells  us  that  most  mammals  must  hear.  It  also  tells  us 


2l6  HISTOLOGY 

that  birds  probably  hear.  Its  value  in  the  case  of  other  vertebrates  is 
negative,  while  for  all  other  creatures  it  is  entirely  valueless.  The 
presence  of  sound-making  apparatuses  in  the  animal  is  rather  poor 
evidence  that  an  auditory  power  is  also  present.  Many  animals  are 
practically  mute,  and  yet  have  the  keenest  of  ears  for  the  sounds  made 
by  enemies. 

The  voice  of  insects  is  often  put  forth  as  evidence  that  they  must 
hear.  The  plecypod  mollusk,  living  on  the  bottom  of  the  stream  where 
the  current  pouring  over  stones  and  sand  must  make  a  noise,  might  use 
the  otocyst  to  hear  the  noise.  But  the  tiny  Cyclas,  living  deep  in  the 
mud  and  ooze  of  the  stillest  ponds,  has  really  no  possible  use  for  its 
otocyst  other  than  to  know  which  direction  is  up  and  which  is  down. 
Most  mollusks  probably  hear  nothing. 

The  presence  of  accessory  tissues  to  gather  and  transmit  the  sound  is 
good  evidence.  The  tympana  and  pinnae  of  various  kinds  tell  unmis- 
takably that  an  auditory  function  is  at  least  a  part  of  the  organ's  duties. 
This  is  the  determining  factor  in  the  frog,  where  experiment  is  uncertain. 
Experiment  is  difficult,  but  some  of  its  positive  results  are  conclusive. 
The  only  reaction  that  we  can  trust  is  a  sudden  motion  or  start  when  the 
sound  is  made,  and  it  is  possible  that  many  forms  would  not  move  even 
if  they  heard  the  noise.  A  too  loud  sound  might  also  stimulate  other 
sense  organs. 

We  shall  study  four  forms  of  tissue  that  can  undoubtedly  perceive 
sound :  the  auditory  hairs  of  the  mosquito  and  other  insects ;  the  chor- 
dotonal  organs  of  an  insect ;  the  ear  of  an  insect,  and  the  ear  of  a  Guinea 
pig,  which  much  resembles  that  of  man  (for  the  possible  auditory  organ 
of  the  cephalopod  mollusks,  see  the  part  on  equilibration). 

The  hair-like  auditory  organs  have  been  best  studied  in  the  antennae 
of  the  male  mosquito  and  in  the  auditory  hairs  of  some  larvae  (Corethra). 
The  mosquito  (a  male)  was  fastened  to  a  glass  slide  by  the  feet  so  that 
he  was  living  and  in  health,  but  quiet  enough  to  be  put  under  the  micro- 
scope and  studied.  Tuning  forks  were  then  sounded  in  a  succession 
of  strong  notes  of  various  pitches,  and  it  was  observed  that  at  certain 
notes  the  hairs  vibrated,  strongest  at  512  vibrations  per  second  and 
weaker  at  some  adjacent  notes  and  in  some  of  the  other  hairs.  The 
vibrations  of  these  hairs  act  upon  a  very  peculiar  and  complex  organ 
found  in  the  second  basal  segment  of  the  antenna.  A  nerve  carries  the 
stimulus  from  this  organ  to  the  brain.  As  this  organ  only  acted  when  the 
hairs  were  at  the  proper  angle  to  the  sound  waves,  and  as  this  angle 
extended  from  directly  in  front  for  some  distance  toward  the  outer  side 
of  each  antenna,  it  can  be  seen  that  the  male  mosquito  can  perceive 
both  the  sound  and  its  direction,  and  thus  can  find  the  female  in  the  dark. 

The  histology  of  this  organ  is,  in  principle,  like  that  of  the  tactile 


AUDITORY   TISSUES 


217 


hairs  of  many  arthropods,  and  also  closely  resembles  the  static  hairs  of 
the  shrimp,  which  we  have  studied,  thus  showing  the  close  relationship 
that  exists  between  these 
three  forms  of  sense  organs 

(Fig.  195). 

The  chordotonal  organs 
in  the  limbs  of  some  katy- 
dids represent  another  differ- 
ent and  somewhat  more 
specialized  form  of  auditory 
apparatus.  It  is  a  wonderful 
organ  because  of  the  many 
kinds  of  highly  differentiated 
tissues  that  cooperate  to  form 
it.  A  very  large  trachea  , 

J        vi  11-  FIG.  195.  —  Longitudinal  section  of  second  antennalseg- 

COmCS  into  the  limb  and  lies  ment  of  a.  mosquito,  MocMonyxculiciformis.  aud.or., 
in  Very  close  Contact  with  One  auditory  organ.  (After  CHILD  in  Zeitschrijt  f.  wiss. 

side  of  it.    The  outer  cuticle 

of  the  limb  becomes  thin  on  an  oval  area  of  this  contact,  and  this  area 
forms  the  tympanum  of  the  auditory  organ.  The  tympanum  is  thus 
made  of  two  thin  layers  of  cuticle  (for  the  trachea  is  a  portion  of  invagi- 
nated  integument)  between  which  lie  the  two  layers  of  simple  epithe- 
lium which  have  formed  them. 

In  the  anterior  of  the  two  widening  spaces,  where  these  two  walls  of 

the  tympanum  sepa- 
rate, lie  the  auditory, 
perceptory  cells  (Fig. 
196).  They  are  of 
two  kinds,  in  which 
the  differences  are 
—sen.c.  mostly  those  of  size 
and  general  form.  It 
is  also  possible,  if 
these  perceptory  cells 
were  derived  from  the 
ectodermal  tissues,  as 
they  probably  were, 
that  the  cells  of  one 
group  originated  from 
the  tracheal  epithe- 
lium, while  the  others 
came  from  that  of  the  outer  integument  of  the  limb.  These  cells  are 
connected  with  some  central  ganglion  by  two  nerves  that  unite  upon 


—  — --sen  c.  or. 


FIG.  196. — Part  of  a  longitudinal,  vertical  section  of  the  fore 
tibia  of  a  young  katydid,  Microcentrum  laurifolium,  10  mm. 
long.  Chordotonal  organ,  sen.c.,  sensory  cells;  sen.c.or.,  cell- 
organ  of  sound  perception;  cu.,  cuticle  of  trachea.  X  1200. 


2l8  HISTOLOGY 

leaving  the  limb.  The  peculiar  structure  of  the  cell  is  remarkable,  and 
is  seen  only  in  the  auditory  organs  of  insects.  Its  cell  body  is  found  in 
the  ganglion  (Siebold's  or  the  supra-tympanal  ganglion  in  this  case), 
and  the  efferent  processes  from  several  of  these  cells  form  the  nerve 
fibers  that  pass  into  the  body.  The  afferent  pole  is  elongated  into  a 
moderately  long  fiber,  on  the  end  of  which  is  seen  the  huge  auditory  end- 
organ  of  the  cell  which  is  often  larger  than  the  cell  body  itself.  Its 
interior  is  occupied  by  space  drawn  out  in  the  axis  of  the  fiber  and  lined 
with  a  cuticular  shell  that  is  open  on  the  end  toward  the  cell  body,  but 
closed  distally  and  thickened  into  a  conical  mass.  The  central  portion 
of  the  afferent  fiber  coming  from  the  cell  is  directed  into  the  open  end 
of  this  peculiar  end-organ,  and  ends  as  a  nerve  fibril  that  projects  freely 
into  the  body.  Its  free  portion  in  the  cavity  is  called  the  axial  filament, 
and  its  vibrations  are  caused  by  the  sound  waves  working  through  the 
tympanum  as  a  medium,  or,  perhaps,  directly.  These  vibrations  produce 
the  stimulation  of  the  nerve  cell.  The  distal  portion  of  this  sensory 
cell-organ  is  further  extended  to  form  a  means  of  attachment  for  the 
cell  to  the  tympanal  surface.  This  is  called  the  terminal  filament.  The 
presence  of  other  nuclei  in  the  scolophores  of  some  insects  might  lead 
one  to  believe  that  the  whole  apparatus  was  not  unicellular,  and  that  other 
cells  than  the  ganglion  cell  had  taken  part  in  the  formation  of  the  scolo- 
phore.  The  axial  filament  at  least  is  a  part  of  this  cell.  When  no 
special  tympanum  exists,  as  in  some  lower  larva?  (see  below),  the  terminal 
filament  may  act  as  a  tympanum  itself. 

As  has  been  said,  there  are  two  groups  of  these  cells.  One  is  known 
as  Siebold's  ganglion,  and  its  end-organs  are  attached  to  the  trachea. 
They  form  a  long  row  of  cells  of  diminishing  length,  and  probably  each 
cell  is  adapted  to  respond  to  a  note  of  different  wave  length.  The  other 
group  is  known  as  the  supra-tympanal  ganglion,  and  its  end-organs  are 
attached  to  the  outer  walls  of  the  limb.  Their  exact  function  would 
make  an  interesting  experimental  study. 

A  somewhat  simpler  organ  of  the  same  kind  exists  in  the  limbs  and 
body  walls  of  many  other  insects.  The  tympanum  may  be  entirely 
lacking,  and  yet  the  chordotonal  organ  be  fairly  well  developed.  This 
condition  is  usually  accompanied  by  a  lack  of  voice  in  the  species,  and 
gives  rise  to  some  doubts  as  to  whether  the  creature  can  hear  or  not. 
If  it  cannot  hear,  we  must  then  decide  as  to  whether  or  not  the  organ  is 
degenerate,  or  a  rudiment  of  one  that  will  be  used  to  hear  later  in  the 
history  of  the  race. 

The  apparently  simplest  form  of  the  chordotonal  organ  is  found  in  the 
body  tissues  of  many  insect  larvae.  In  this  form,  as  exemplified  by  the 
structure  of  the  larva  of  a  fly,  Chironomus,  the  auditory  cells  occur  in 
many  segments  of  the  body  in  small  groups  of  from  one  to  three  or  more. 


AUDITORY   TISSUES 


219 


The  terminal  filament  is  long  and  slender,  and  is  attached  to  some  part 
of  the  body  cuticle.  This  brings  a  tension  upon  the  central  ganglion. 
In  many  cases  a  ligament  is  devel- 
oped leading  from  another  part  of 
the  body  wall  and  attached  to  the 
ganglion  cell  mass.  The  short 
length  of  this  ligament  brings  the 
tension  between  two  parts  of  the 
body  wall  in  the  same  segment 
and  relieves  the  delicate  nerve 
and  ganglion,  as  well  as  the  cen- 
tral ganglion,  of  nearly  all  strain. 
At  the  same  time  it  allows  the 
ligament-filament  cord  to  vibrate 
under  tension,  and  thus  stimulate 
the  axial  filament  which  lies  in 
the  scolophore  in  the  middle  sec- 
tion of  the  compound  tympanic 
cord  (Fig.  197). 

On  account  of  its  lack  of  an  in- 
ternal air  space  and  an  externally 
differentiated  tympanum,  we  must 
look  upon  this  last  auditory  organ 
as  the  simplest  insect  form,  especially  that  one  which  lacks  a  ligament. 

One  more  insect  form  should  be  briefly  examined,  as  exhibiting  the 
most  highly  specialized  form  of  the  insect's  auditory  organ.  This 
is  the  "  ear  "  of  the  "  grasshopper  "  or  locust.  This  organ  consists 
essentially  of  the  same  auditory  cells,  with  their  terminal  filaments  rest- 
ing against  a  very  large  tympanum.  The  tympanum  is  stretched 
across  the  enlarged  opening  of  an  abdominal  trachea  that  it  closes, 
with  the  exception  of  a  small  pore  left  to  allow  of  an  equal 
distribution  of  air  pressure.  This  allows  the  membrane  to  vibrate 
freely.  Resting  against  the  tympanum,  and  attached  by  their  terminal 
filaments  to  two  horny  irregularities  on  its  surface,  are  the  auditory 
nerve  cells  or  scolophores.  The  nerve,  which  comes  from  the  ganglion 
formed  by  their  assembled  cell  bodies,  passes  over  the  inner  surface 
of  the  tympanum  and  into  the  body,  where  it  enters  a  central  ganglion. 

A  strong  peculiarity  of  all  the  above  insect  auditory  organs  is  the 
small  number  and  high  specialization  of  the  auditory  nerve  cells  or 
scolophores,  which,  with  the  possible  exception  of  the  mosquitoes,  are 
found  in  all  of  them.  In  the  mosquito  the  perceptory  nerve  cells  are 
found  in  the  second  basal  joint  of  the  antenna,  where  they  form  a  peculiar 
ganglion  (see  Fig.  195). 


larva,  nv.,  nerve;  lig.,  ligament;  fil.,  "ter- 
minal filament "  or  cell-organ  of  sound  percep- 
tion; aud.c.,  auditory  cells.  (After  GRABER  in 
Arch.f.  mik.  Anat.) 


22O  HISTOLOGY 

Most  of  the  vertebrate  animals  have  a  sense  of  hearing  which  is 
made  possible  by  the  possession  of  auditory  sense  cells  and  all  the 
accessory  tissues  necessary  to  gather  and  intensify  the  sound  waves 
and  transmit  them  to  the  perceptory  cells. 

The  perceptory  cells  in  this  case  have  been  developed  or  evolved  from 
the  static  epithelium  or  from  a  common  epithelium  from  which  both  of 
these  have  originated.  In  the  few  other  forms  of  animals  that  can  hear, 
the  auditory  tissues  have  originated  from  entirely  different  parts  of  the 
body. 

The  process  of  the  evolution  of  auditory  cells  from  the  tactile  or 
static  epithelium  of  the  internal  ear-sac  is  one  whose  progressive  steps 
can  apparently  be  traced  in  the  taxonomic  series  of  vertebrates.  Fishes 
apparently  are  just  coming  into  their  power  of  hearing,  and  it  is  a  question 
if  they  can  hear  or  not.  Some  probably  can  hear  a  few  low  sounds. 
From  the  fishes  up,  the  series  of  amphibia,  reptiles,  birds,  and  mammals 
show  successively  higher  stages  in  the  development  of  a  part  of  the  ear-sac 
into  a  cochlea  or  region  of  auditory  perception,  until,  when  we  arrive  at 
the  mammals,  we  find  the  beautifully  arranged  series  of  auditory  cells 
and  accessory  supporting  cells  which  are  grouped  in  a  row  which  winds 
in  a  spiral  to  save  space.  Like  a  snail  shell,  this  hollow,  spiral,  tubular 
part  of  the  sacculus  diminishes  in  size,  and  the  resulting  different  lengths 
of  cells  probably  perceive  lower  or  higher  notes  of  sound. 

The  auditory  epithelium  of  the  Guinea  pig  will  serve  as  an  example, 
and  we  shall  study  the  structure,  although  we  cannot  hope  to  entirely 
understand  the  mechanism.  Only  the  membranous  portion,  and  espe- 
cially its  specific  epithelium,  will  be  treated  of  in  this  description,  and 
we  shall  begin  by  short  embryological  explanation. 

The  whole  epithelium  under  consideration  was  originally  a  part  of 
the  body  epithelium  on  the  sides  of  the  head.  At  an  early  period  (10 
days  in  the  rabbit)  this  epithelium  thickened  and  was  invaginated  into 
a  sac  with  a  narrow  duct  connecting  it  with  the  exterior. 

The  sac  continued  to  enlarge  and  the  duct  to  close  until  it  was  cut 
off  entirely  and  obliterated.  This  left  the  sac  as  an  internal  cavity  lined 
with  an  epithelium.  The  sac  enlarged  and  constricted  in  the  middle 
until  it  formed  two  sacs  united  by  a  duct.  These  two  compartments 
are  called  utriculus  and  sacculus,  while  the  duct  is  known  as  the  utriculo- 
saccular  duct  in  the  adult. 

The  utriculus  now  evaginated  from  its  sides  the  three  semicircular 
canals,  three  curved  tubes  opening  into  the  sac  with  both  ends.  Each 
tube  was  enlarged  into  an  ampulla  which  is  described  under  the  static 
tissues.  The  epithelium  of  different  parts  of  both  sacculus  and  utriculus 
were  differentiated  in  several  regions,  while  the  whole  complicated  organ 
was  encased  in  a  bony  covering  which  formed  around  it. 


AUDITORY  TISSUES  221 

Meanwhile  the  sacculus  had  evaginated,  from  its  inner  epithelial 
surface,  a  long  tube  which  has  curled  into  the  snail- shell- shaped  structure 
mentioned  above.  This  is  the  cochlea,  whose  rudiment  is  to  be  seen  as  a 
small  evagination  from  the  inner  surface  of  the  sacculus  in  the  lower 
vertebrates,  which  has  become  elongated  for  some  distance  by  continued 
invagination  in  the  birds,  where  it  is  straight  or  partly  curved,  and  which 
is  thus  curled  up  in  the  mammals  on  account  of  its  great  length  and 
development. 

This  spiral,  membranous  tube  does  not  retain  a  round  shape,  nor 
does  it  fill  the  rounded  bony  cavity  which  is  provided  for  it  in  the  Guinea 
pig.  As  is  best  seen  in  a  transverse  section  of  one  of  its  coils,  it  is  applied 
by  a  third  of  its  circumference  to  a  narrow  area  of  the  outer  wall  of  this 
bony  case,  and  is  met  by  a  bony  and  muscular  septum  or  shelf  that  reaches 
out  from  the  inner  wall  to  form  a  contact  with  a  second  third,  which  thus 
lies  at  about  right  angles  to  the  first  mentioned.  The  last  third  of  the 
epithelium-bearing,  membranous  tube  is  stretched  through  the  cavity 
as  a  septum  which  divides  the  triangular  interior  of  the  membranous 
tube  from  the  large  part  of  the  bony  tube  which  the  membranous  cochlea 
occupies. 

In  section,  then,  the  membranous  cochlea  is,  roughly,  an  equal-sided 
triangle  with  one  side  applied  to  the  wall  of  the  bony  tube,  one  side  to  the 
septum  that  divides  the  bony  tube  into  two  parts,  and  the  other  stretched 
across  the  upper  division  of  the  bony  tube.  On  the  first  and  third  of  these 
the  epithelium  is  of  no  further  interest  to  us,  and  we  shall  study  that  part 
which  rests  on  the  septum,  for  it  is  here  that  the  auditory  apparatus  is 
formed  out  of  the  layer  of  epithelial  cells.  The  apparatus  forms  a  band 
which  is  cut  several  times  in  its  spiral  course  by  a  median  section  through 
the  long  axis  of  the  cochlea. 

Figure  198  represents  this  basal  section  of  the  epithelium  of  the 
membranous  tube,  as  cut  in  the  second  spiral  of  a  young  Guinea  pig's 
cochlea.  The  apparatus  is  called  the  organ  of  CorlL 

Beginning  from  right  to  left,  we  find  that  the  simple  epithelium  is 
cuboidal  where  it  first  appears  on  the  septum.  These  cells  have  been 
called  the  cells  of  Claudius.  They  rise  up  against  one  of  their  neigh- 
bors which  is  grown  to  five  or  six  times  their  height  and  has  acquired  a 
pointed  end.  This  cell  is  known  as  Henserfs  cell,  and  while  but  one  of 
them,  representing  a  single  row,  is  seen  in  the  specimen  from  which 
our  drawing  is  taken,  they  form  a  double  row  of  cells  in  the  Guinea  pig 
and  several  rows  in  extent  in  the  cat  and  some  other  mammals. 

The  next  six  cells  (representing  six  rows)  are  of  two  kinds  placed 
alternately.  Three  of  them,  including  the  first,  are  tall  supporting  cells 
with  narrow  upper  bodies  that  expand  at  the  tip  into  plates.  These 
plates  reach  from  one  cell  to  the  other  and  to  the  tip  of  the  Hensen  cells 


222  HISTOLOGY 

to  form  an  almost  continuous  cover.  These  supporting  cells  are  known 
as  the  Deiter's  cells.  This  cover  is  interrupted  at  regular  intervals  to 
permit  the  tops  of  the  short  outer  auditory  cells  or  outer  cells  to  form 
a  part  of  the  surface  in  the  row.  These  short  cells  do  not  reach  to  the 
basement  membrane,  but  are  supported  in  their  elevated  position  by 
the  three  rows  of  Deiter's  cells. 

The  next  cells  appear  in  two  rows,  which  are  composed  of  the  tallest 
cells  of  all.  They  spring  from  broad  bases  wide  apart  and  lean  toward 
each  other  at  an  angle  with  narrow  bodies  which  touch  and  merge  by 
a  wide,  curved  contact  at  the  top.  The  first  of  these  two  rows  are  the 
outer  and  the  second  the  inner  pillar  cells.  The  round  tube-like  space 


FIG.  198.  — Section  of  the  organ  of  Corti  of  a  young  Guinea  pig,  Cavia.  d.c.,  cells  of  Claudius; 
h.c.,  Hensen's  cells;  d.c.,  Deiter's  cells  or  supporting  cells  of  the  sensory  epithelium; 
aud.c.,  auditory  cells  or  hair  cells  (outer) ;  p.c.,  outer  and  inner  pillar  cells;  i.aud.c.,  inner 
auditory  cell  or  hair  cell;  n.fi.,  nerve  fibrils;  m.t.,  membrane  tectoria;  s.s.c.,  cells  lining 
the  sulcus  spiralis. 

that  runs  between  them  is  the  tunnel  of  Corti.  The  greater  part  of  the 
cell  body  is  a  specialized  product  of  the  cytoplasm. 

Next  to  the  inner  pillar  cells  is  found  a  single  row  of  hair  cells,  the 
inner  hair  cells,  or,  as  we  shall  call  them,  the  inner  auditory  cells.  They 
are  followed  by  a  portion  of  simple  epithelium  that  lines  a  groove 
called  the  sulcus  spiralis.  This  epithelium  is  thicker  than  that  which  is 
continued  over  the  remainder  of  the  septum  and  across  as  the  free  mem- 
brane veslibularis  to  the  outer  bony  wall  and  thence  to  the  point  at 
which  we  began,  the  cells  of  Claudius. 

The  nerve  supply  consists  of  the  different  processes  of  neurons  lying 
in  a  ganglion  that  is  found  in  the  immediate  neighborhood.  These 
fibers  pass  along  through  the  bony  septum  which  is  called  the  lamina 
spiralis  and  send  naked  fibrils  in  a  bundle  to  the  hair  cells.  Some  of  these 
fibrils  form  perceptory  end-plates  on  the  inner  hair  cells  and  the  rest 
cross  the  tunnel  of  Corti,  and,  passing  into  the  spaces  between  the  sus- 
tentacular  cells  and  under  the  hair  cells,  end  in  the  same  way  on  the  outer 
hair  cells. 


AUDITORY    TISSUES  22$ 

We  have  here  ample  proof  that  the  hair  cells,  both  outer  and  inner, 
are  the  sensory  auditory  cells.  The  stiff,  short  rods  set  in  their  upper 
surfaces  are  the  cell-organs  of  auditory  perception,  and  receive  the 
stimulus. 

Our  next  concern  is  to  know  as  well  as  we  can  how  this  stimulus 
is  imparted  to  the  auditory  rods.  Do  the  waves  of  sound  vibrate  them, 
or  is  there  an  intermediate  tissue  or  substance  ? 

As  in  the  tactile  and  static  tissues,  we  find  that  here  an  intermediate 
substance  is  present  and  is  probably  necessary.  This  is  a  plate  or  shelf 
of  material  which  projects  from  the  elevation  on  the  left,  the  labium 
veslibularis,  and  reaches  across  to  cover  the  hair  cells  with  its  edge.  It 
is  called  the  membrana  tectoria  and  is  probably  a  cuticular  product  of  the 
labial  cells.  Such  a  broad  surface  must  easily  be  made  to  vibrate  by  the 
waves  of  sound,  especially  as  they  come  intensified  by  accessory  tissues, 
and  must  thus  play  mechanically  upon  the  auditory  rods  and  give  them 
a  characteristic  stimulus. 

The*  accessory  tissues  are  the  pinna,  a  shell-shaped  organ  designed  to 
catch  a  volume  of  sound  waves,  and,  having  concentrated  them,  to  pro- 
ject them  through  a  tube  and  against  a  stretched  membrane,  the  tym- 
panum. In  a  frog  this  tympanum  is  larger  and  directly  exposed  to  the 
air  waves  without  the  aid  of  such  accessory  tissues. 

The  tympanum  is  composed  of  both  connective-tissue  and  epithelial 
elements,  and  the  four  layers  are,  from  without  inward,  a  thin  stratified 
epithelium  called  the  stratum  cutaneum;  a  layer  of  connective-tissue 
fibrils  arranged  as  a  radiating  tendon  and  known  as  the  stratum  radia- 
tum ;  another  connective-tissue  layer  called  the  stratum  circulare ;  and  an 
internal  layer  of  simple  cuboidal  epithelium  that  is  continuous  with  the 
epithelium  lining  the  middle  ear.  The  three  tiny  bones  which  form  a 
chain  to  transmit  the  vibration  to  the  internal  ear-sac  are  composed  of 
a  very  fine  and  dense  bone  tissue. 

Technic.  —The  same  remarks  as  to  technic  may  be  applied  here  as 
were  found  following  the  previous  part.  It  may  be  added  that  no  care- 
ful studies  of  a  tissue  of  hearing  can  be  satisfactory  if  they  are  not 
grounded  on  experimental  work  that  demonstrates  the  tissues  to  be 
actually  sensitive  to  sound  stimuli.  The  auditory  tissues  of  insects  are 
very  hard  to  handle  by  the  section  method,  owing  to  the  chitinous  struc- 
tures with  which  they  are  associated.  When  the  insects  are  small  and 
have  a  delicate  cuticle,  the  sections  may  be  easily  secured.  Also  when  a 
larger  insect  with  a  heavy  cuticle  has  just  emerged  from  the  molt,  its 
shell  is  soft  and  may  be  ignored.  Chitin  can  be  softened,  but  always 
at  the  expense  of  the  other  tissues.  It  is  better  to  remove  the  chitin  if 
possible. 


224  HISTOLOGY 

LITERATURE 

CHILD,  C.  M.    "Ein  bischer  wenig  beachtetes  antennales  Sinnesorgan  der  Insecten,"  etc., 

Zeits.  f.  Wiss.  Zool.,  Band  LVIII,  1894,  pp.  478-528. 

HENSEN,  V.     "  Uber  das  Gehororgan  von  Locusta,"  Zeits.  f.  Wiss.  Zool.,  Band  XVI,  1866. 
GRABER,  VITUS.    "Die  chordotonalen  Sinnesorgane  und  das  Gehor  der  Insecten,"  Arch. 

f.  mik.  Anat.,  Band  XX,  p.  506. 
DENKER,  A.     "Zur  vergleichenden  Anatomic  des  Gehororgans  der  Saugertiere,"  Erg. 

Anat.  u.  Entwick,  Band  IX,  1899. 
STREETER.     On  the  Development  of  the  Membranous  Labyrinth  and  the  Acoustic  and 

Facial  Nerves  in  the  Human  Embryo.     Am.  Journ.  of  Anatomy,  Vol.  VI,  Part  2. 
SMITH,  G.     "The  Middle  Ear  and  Columella  of  Birds,"  Quart.  J.  Mic.  Science,  Vol. 


.. 
KISHI,  J.     "Uber  den  peripheren  Verlauf  und  die  Endigung  des  Nervus  Cochleae,"  Arch. 

f.  mik.  Anat.,  Band  LIX,  1902. 
RETZIUS,  G.     "Die  Endigungsweise  des  Gehornerven,"  Biol.  Unters.,  1892  und  1893. 


TISSUES   OF  LIGHT  PERCEPTION 

Light  is  produced  by  short,  rapid  undulations  of  the  invisible  ether. 
These  movements  are  capable  of  producing  chemical  and  physical 
changes  in  living  and  dead  matter.  Their  effect  on  ordinary  animal 
cells  is  not  perceptible  at  the  nerve  centers.  By  some,  they  are  thought 
to  be  a  stimulant  and,  in  too  great  quality  or  intensity,  a  poison  to  ordi- 
nary protoplasm.  Thus  the  surface  of  most  animal  bodies  is  fitted  to 
keep  the  light  from  all  underlying  parts  (see  Chapter  XIII,  Pigment). 

Upon  certain  of  the  body  cells,  however,  light  does  leave  a  definite 
impression.  There  are  some  nerve  cells  that  not  only  perceive  the  light 
but  are  stimulated  by  it  to  send  a  report  of  the  fact  as  an  impulse  to  a 
nerve  center,  either  through  their  own  efferent  process  or  through  com- 
municatory nerve  cells  that  form  a  path.  Such  cells  are  known  as  the 
visual  cells,  relinulte,  rod-cells,  etc.  We  shall  call  them  the  visual  cells. 
The  light  is  given  access  to  them  through  accessory  tissues  made  trans- 
parent for  the  purpose. 

The  visual  cell,  in  its  specialized  state,  has  developed  a  specific  cell 
organ,  a  peculiar,  rod-like  structure  secreted  or  otherwise  formed  by 
the  cytoplasm  and  capable  of  being  stimulated  by  the  ether  waves.  It 
is  weakly  developed  and  almost  invisible  in  some  low  sight  cells,  while 
in  others  it  is  larger  than  the  cell  which  produced  it,  and  so  clearly  dif- 
ferentiated that  it  appears  to  be  a  separate  structure.  It  is,  where  care- 
ful observations  have  been  made,  laminated,  and  the  separate  plates  or 
rods  of  which  it  is  composed  usually  lie  at  right  angles  to  the  light  which 
stimulates  it.  Its  exact  chemical  and  physical  relations  to  the  light 
waves  during  stimulation  are  not  known.  It  can  be  stimulated  by  other 
factors  than  the  light  waves,  as  pressure  and  chemical  activity,  but  it 
gives,  under  these  circumstances,  the  same  impression  to  the  brain 


VISUAL    TISSUES  22$ 

centers  as  if  it  were  affected  by  light  waves.  It  cannot  perceive  all 
light  waves,  and  varies  considerably  in  the  number  of  kinds  of  them  that 
can  stimulate  it.  This  cell-organ  of  sight  is  known  by  various  names,  of 
which  we  shall  use  but  two,  the  rhabdome  or  the  visual  rod. 

The  rhabdome  is  placed  in  various  positions  on  the  cell  body.  It 
may  be  on  the  edge  or  on  the  end  of  the  cell  and  may  assume  any  position 
of  its  body  to  fit  in  with  other  optic  structures.  More  rarely  it  is  found 
inside  the  cell.  It  may  even  be  formed  upside  down  in  case  the  cell 
receives  its  light  rays  from  behind  (eye  of  man  and  Pecten).  It  is  not 
known  if  the  rays  of  light,  striking  the  plates  from  their  rear  or  from  any 
other  direction  than  in  the  front,  can  stimulate  the  cell. 

The  visual  cells  are  sometimes  found  scattered  on  the  body  surface, 
but  are  usually  collected  into  one  or  two,  or  even  many,  groups  which, 
together  with  the  accessory  tissues,  are  called  the  eyes.  Eyes  may  per- 
form three  functions  for  their  possessor :  to  perceive  the  light  according 
to  its  intensity,  or  to  perceive  it  according  to  its  direction,  or  to  record 
light-images  of  the  objects  from  which  the  rays  come.  Some  eyes  do 
all  of  these  things. 

To  perceive  the  light  alone  according  to  its  intensity  is  a  function 
of  the  individual  visual  cell,  as  well  as  of  the  highest  eyes.  To  determine 
its  direction  depends  upon  the  position  of  the  visual  cell  with  reference 
to  the  body  or  some  larger  part  of  the  body.  It  is  also  determined  by 
the  position  of  a  cell  or  group  of  cells  stimulated  to  the  exclusion  of  the 
rest  of  the  retina.  To  perceive  an  image  depends  upon  the  relations 
of  many  neighboring  visual  cells  to  each  other  and  the  presence  of 
properly  arranged  brain  centers  that  can  receive  their  numerous  reports 
as  a  related  whole. 

Some  forms  of  visual  cells  probably  exist  that  have  not  been  pointed 
out  to  science.  The  surface  of  the  body,  in  some  animals,  contains 
other  light-sensitive  cells  than  those  in  the  eyes,  and  these  can  perceive 
light  and  even  the  direction  of  light.  The  frog  forms  a  concrete  exam- 
ple. If  deprived  of  its  eyes,  a  frog  will  still  be  able  to  orient  itself  with 
reference  to  a  ray  of  light. 

The  most  primitive  form  of  visual  organ  would  consist  pf  certain 
simple  epithelial  cells,  different  from  their  fellows  only  in  their  develop- 
ment of  a  visual  cell-organ  or  region  and  their  consequent  ability  to 
perceive  light.  An  organ  of  this  kind  probably  exists  in  such  animals 
as  the  earthworm,  the  frog,  and  some  medusae.  This  brings  us  once  more 
to  realize  that  very  simple  eyes  will  sometimes  be  met  with  in  highly 
organized  animals.  We  shall  learn  later  that  some  simple  or  lowly 
organized  animals  have  quite  complex  eyes,  and  further,  that  both  kinds 
of  eyes  may  exist  in  the  same  animal. 

Pigment  is  almost  always  found  in  connection  with  the  visual  cells. 
Q 


226  HISTOLOGY 

Its  function  seems  to  be  the  secondary,  but  important,  one  of  protecting 
the  visual  cells  or  their  rods  from  undue  amounts  of  light  as  well  as  form- 
ing an  absorbent  background  that  will  free  them  from  unnecessary  re- 
flections. This  pigment  is  sometimes  in  the  visual  cells  themselves,  but 
more  often  in  other  cells  that  are  specialized  for  this  function  and  found 
in  close  connection  with  the  visual  cells.  It  appears  as  the  usual  clouds 
of  fine,  dark  brown  granules  in  the  cytoplasm  of  the  cells. 

Some  tissue  cells  in  the  neighborhood  of  the  visual  cells  are  modified 
so  that  their  entire  body  becomes  exceedingly  transparent  and  of  a  high 
index  of  refraction.  These  cells  also,  either  individually  or  collectively, 
assume  a  spherical  or  other  curved  form  which  serves  to  collect  and  thus 
concentrate  the  power  of  the  rays  or  even  to  arrange  them  as  an  image 
on  the  layer  of  visual  cells.  Such  a  refractive  body  is  the  lens  of  the  eye. 
The  lens  is  not  always  a  cellular  structure.  Many  consist  of  transparent 
cell-products,  as  the  cuticular  lens  of  the  arthropod  eye  or  the  gelatinous 
lens  in  the  visual  organs  of  some  worms. 

In  a  few  rare  cases  the  lens  as  an  image  former  is  done  away  with 
entirely,  and  a  diaphragm  in  front  of  the  retina  is  used  to  produce  an 
image,  on  the  principle  of  the  pin-hole  camera.  This  diaphragm  also 
serves  to  determine  the  amount  of  light  to  be  admitted  to  the  retina,  and 
in  this  capacity  it  becomes  known  as  an  iris.  A  well-developed  iris  is 
found  in  some  eyes  that  also  have  a  lens,  and  it  serves  here  to  regulate  the 
amount  of  light  that  shall  enter.  It  does  not  take  part  in  the  image  for- 
mation when  a  lens  is  present. 

When  arranged  to  receive  an  image,  the  visual  cells,  together  with 
some  accessory  cells,  are  known  as  a  retina.  Many  communicatory 
nerve  cells  are  included  in  this  structure.  They  sometimes  form  part 
of  the  layer  and  sometimes  are  removed  from  it  in  ganglia  where  they 
have  a  peculiar  arrangement  in  layers  that  evidently  have  some  relation 
to  the  retina. 

The  above  structures  are  all  very  delicate  and,  for  the  most  part,  must 
be  protected  from  contact  with  the  exterior.  For  this  purpose  we  find 
some  cells  that  make  it  their  duty  to  form  protecting  coverings  that 
shield  these  delicate  structures.  Such  an  organ  is  known  as  a  cornea, 
and  may  be  composed  of  a  single  layer  of  epithelium,  or  of  an  epithelium 
lying  on  a  connective  tissue  or  of  no  cells  at  all,  as  in  the  arthropods,  where 
it  is  the  chitinous  secretion  of  certain  outer  cells.  One  prominent  feature 
of  all  cells  and  tissues  that  take  part  in  the  formation  of  a  cornea  or  a 
lens  is  their  transparency.  The  necessity  for  this  is  apparent. 

Visual  organs  in  the  Protozoa.  — The  Protozoa  that  are  unques- 
tionably animals  show  no  clearly  defined  light- perception  region.  The 
general  surface  of  the  cell  is  sensitive  to  certain  intensities  of  light,  the 
dorsal  surface  probably  being  more  sensitive  than  the  ventral  region. 


VISUAL    TISSUES 


227 


Certain  unicellular  forms,  mostly  probably  plants,  have  light- perceiving 
structures.  Here  a  certain  region  becomes  furnished  with  a  disk  or 
cup- shaped  pigment  spot  and  a  cuticular  lens  (Fig.  199).  In  this  region 
the  cytoplasm  is  more  highly  sensitive  to  light.  This  is  primarily  a 
structure  for  perceiving  the  intensity  of  the  light.  But,  as  it  is  always 
placed  eccentric  to  the  long  axis  of  the  plant,  and,  since  the  plant  moves 
about,  rotating  on  its  long  axis,  it  becomes  also  a  device  for  perceiving 
direction  of  light  rays.  In  colonial  forms,  presenting  these  so-called 


FIG.  199.  — Individual  of  the  flagellate,  Chlamydomonas  reticulata;  e.,  eye-spot  with  pigment 
and  lens;  nu.,  nucleus;  c.v.,  contractile  vacuole.     x  1000. 

stigmata,  the  stigmata  are  always  arranged  with  reference  to  aiding  the 
colony  to  determine  the  direction  of  the  light  source. 

We  can  find  no  homogeneous  basis  upon  which  to  classify  the  visual 
organs  of  the  Metazoa,  owing  to  the  way  in  which  the  prominent  features 
of  these  organs  are  distributed  among  the  various  examples.  There 
seems  to  be  very  little  homology  among  them  based  upon  a  common 
ancestry,  and  the  same  animal  will  often  have  two  different  kinds  of  eyes 
on  different  parts  of  its  body,  or  even  near  one  another  on  the  same  part. 
We  shall  therefore  consider  them  in  groups  that  are  rough  associations 
of  eyes  of  somewhat  the  same  degree  of  tissue  complexity,  or  else  which 
belong  to  closely  related  groups  of  animals.  Comparisons  of  these,  from 
the  tissue  standpoint,  form  an  interesting  study.  We  shall  begin  our 
study  of  the  eye  of  Metazoa  with  a  very  simple  eye. 

The  eye  spot  of  a  plecypod  mollusk,  Solen,  as  described  by  Sharp,  is 
probably  the  simplest  true  visual  organ  that  has  a  demonstrable  struc- 


228 


HISTOLOGY 


ture  (Fig.  200).  The  epithelium  on  the  mantle  edge  showed  pigmented 
areas.  As  pigment  is  so  often  associated  with  light  perception,  experi- 
ments were  tried,  and  it  was  found  that  the  mollusk  reacted  to  light  on 
these  spots  and  on  them  only. 

Sections  reveal  a  thickening  of  the  epithelium  due  to  the  simple 
lengthening  of  the  columnar  cells.  Their  distal  ends  have  acquired  a 
rather  heavy  mass  of  black  or  dark  brown  pigment.  Otherwise,  no 
specific  rhabdome  is  to  be  seen. 

A  feature  of  this  spot  which  helps  to  decide  that  it  is  an  eye  is  the 
transparent  thickening  of  the  very  slight  cuticle  which  is  formed  in  this 
region,  into  a  flat  lens.  This  lens  is  arranged  so  as  to  rather  weakly 
concentrate  the  light  rays  on  the  most  pigmented  cells.  A  form  without 
this,  cuticular  lens  would  represent  the  very  simplest  eye. 


—  -I. 


FlG.  200.  — Vertical  section  of  an  eye-spot  on  the  mantle  edge  of  Solen  vagina;  I.,  lens.     (After 
BENJAMIN  SHARP.) 

The  eyes  of  some  starfishes  furnish  still  other  examples  of  a  very 
primitive  light-perceiving  organ.  These  forms  also  show,  among  their 
different  species,  a  gradual  succession  of  stages  which  may  be  considered 
to  indirectly  represent  phylogenetic  steps  in  the  development  of  the 
echinoderm  eye.  A  very  simple  form  is  to  be  seen  in  Astropecten 
Mulleri  (Fig.  201).  In  this  animal  the  simple  epithelium  of  the  body 
is  modified,  over  a  portion  of  the  radial  nerve  called  the  eye  cushion,  so 
that  it  consists  of  two  kinds  of  cells. 

The  first  are  the  supporting  cells  (sup.c.,  Fig.  201)  which,  besides 
acting  as  support  for  the  surrounding  tissues,  are  used  to  produce  from 
their  distal  ends  the  thin,  double-layered,  outer  cuticle  (cu.\  Proxi- 
mally  they  rest  on  a  basement  membrane  (b.  m.)  which  separates  them 
from  the  underlying  connective  tissue  and  muscle. 

Placed  between  these  supporting  cells,  either  singly  (rare)  or  in 
groups  of  from  two  to  five  or  more,  as  seen  in  the  section,  are  the  other 
modified  epithelial  cells,  the  light-perceiving  cells  or  visual  cells  (vis.  c.}. 
They  are  stouter  but  more  delicate  in  texture,  and  the  nucleus  is  placed 
as  a  rule  farther  distally  in  the  cell  than  was  the  case  in  the  supporting 
cell.  The  distal  end  does  not  rest  against  the  cuticle,  but  projects  as  a 


VISUAL    TISSUES 


229 


rounded  end  into  the  space  between  the  cuticle  and  a  parallel  inner 
membrane  called  the  limiting  membrane.  This  space  is  filled  with  a 
fluid  in  life,  and  the  ends  of  the  sensory  cells  which  pass  into  it  through 
the  limiting  membrane  are  the  cell-organs  of  light  perception  or  visual 
rods. 

The  proximal  ends  of  the  visual  cells  do  not  reach  to  the  basement 
membrane,  but  are  prolonged  into  delicate  nerve  fibers  to  conduct  the 
impulses  to  the  nerve  centers.  These  nerve  fibers  are  seen  in  the  draw- 
ings as  sections  of  numerous  fibers  which  lie  among  the  base  of  the  tall 
and  thin  supporting  cells. 


FIG.  201.  —  Parts  of  retinal  tissue  from  the  eyes  of  (.4),  Astropecten  MiUleri  and  (J5),  Aslerias 
tennispina;  vis.c.,  visual  cells;  sup.c.,  supporting  cells;  /.,  lens;  CM.,  cuticle;  b.m.,  basement 
membrane ;  l.m.,  limiting  membrane ;  nv.f.,  nerve  fiber  layer.  (After  PFEFFER.)  x  about  600. 


This  eye  has  no  lens  and  is  of  extreme  simplicity.  Drawn  in  the 
same  figure  with  it  is  a  representation  of  an  eye  from  another  starfish, 
Asterias  tennispina.  On  the  optic  cushion  of  this  echinoderm  the 
visual  cells  are  not  distributed  diffusely  as  in  the  preceding  example, 
but  are  collected  into  groups  to  form  more  definite  "  eyes."  Each  of 
these  groups  is  depressed  into  a  cup-like  hollow,  carrying  the  limiting 
membrane  with  it  and  displacing  to  some  degree  the  surrounding  sup- 
porting cells.  Such  of  these  latter  as  immediately  surround  the  depres- 
sion bend  over  and  touch  the  inner  surface  of  the  cuticle,  and,  besides 
forming  and  supporting  this  cuticle,  they  deposit  on  its  inner  surface 
a  delicate,  flat  lens  (I.).  The  lumen  of  this  depression  or  invagination 
remains  open  and  is  filled,  in  life,  with  a  fluid.  The  sensory  cells  form 


230 


HISTOLOGY 


nerve  fibers  from  their  proximal  ends,  and  these  fibers  unite  to  form 
the  same  nerve  tract  that  was  to  be  seen  in  Astropecten. 

It  should  be  noticed  that 
these  eyes  are  slight  ad- 
vances on  the  eye  of  Solen, 
because  of  the  development 
of  a  visible  rhabdome  or 
visual  rod  on  the  sensory 
cell.  In  both  of  the  above 
visual  organs  the  light  strikes 
the  perceptory  cell  directly, 
and  from  a  distal  position. 
One  more  of  the  extremely 
simple  eyes  should  be  studied 
in  a  medusa,  Charybdea  mar- 
supialis.  This  animal  has 
two  very  different  kinds  of 

FIG.  202.  —  Eye  of  Charybdea  marsupialis.     4,  general  eves    On    One    and    the    Same 

view;  B,  greater  magnification  of  four  cells  to  show  '            r    v     U     1          TU 

the   alternation   of   sensory   cells  (s.c.)  with  pigment  Part  of    lts   bocly-       -1  he  &}m~ 

cells   (pg.c.);    sur.,   outer  surface;   A  X  760.      (After  plest    is  shown,   in  a  Vertical 

section,  by  Figure  202.  Here 

we  again  find  the  pigment  cells  having  very  much  the  same  appearance 
as  they  had  in  Solen.  But  the  point  to  be  noticed  is  that  these  pig- 
ment cells  are  not 


sensory    cells, 
function   hav- 


the 
this 

ing  been  left  to  al- 
ternate visual  cells 
which  have  devel- 
oped a  sight  rod 
for  the  purpose. 
These  visual  cells 
are  differentiated 
out  of  the  same 
primitive  epithe- 
lium from  which 
the  pigment  cells 
were  derived,  and 
two  of  them  are 
pictured  in  the  fig- 
ure, much  enlarged 


FIG.  203.  —  Section  of  the  double  eye  of  Aurelia  aurita.  s.c.,  visual 
cells;  pg.c.,  pigmented  cells;  ec.,  ectoderm;  en.,  endoderm;  mes., 
mesoglcea;  x  760.  (After  SCHEWIAKOFF). 


and  almost  in  their  natural  relations  to  the  pigment  cells,  a  slight 
space    being    left   for   the    sake   of   clearness.     The   complex  eye   of 


VISUAL    TISSUES 


231 


Charybdea  we  shall  not  describe,  its  type  being  represented  later  by 
other  forms. 

Another  medusa,  however,  has  an  intermediate  type  which  should 
be  considered  here.  This  form,  Aurelia  aurita,  has  one  part  of  its  body- 
wall,  consisting  of  all  three  layers  (the  mesoglcea  weakly  represented), 
developed  into  an  eye  that  is  a  simple  inverted  type.  That  is  to  say,  the 
light  passes  through  the  cells  and  strikes  their  visual  cell-organ  from 
a  proximal  position.  This  eye  is  pictured  in  Figure  203,  and  we  see  that 
a  portion  of  the  animal's  ectoderm  has  been  proximally  produced  in 
two  places  to  form  two  knobs,  one  of  which  is  larger  than  the  other. 
These  knobs  are  composed  of  long  sensory  cells  of  ectodermal  origin, 
which  rest  on  the  inner  surface  of  a  pocket-like  layer  of  cells  which  have 
been  invaginated  distally  (or  evaginated  if  they  are  considered  with 
reference  to  their  distal  surface)  to  form  these  pockets. 

The  cells  which  line  the  pockets  have  developed  pigment  in  their 
proximal  ends  (these  ends  are  di- 
rected distally  with  reference  to  the 
animal's  exterior),  and  form  the  pro- 
tective and  absorptive  layer  of  the 
eye.  There  is  no  lens  connected  with 
this  eye,  although  some  medusae  have 
a  well-developed  one.  The  mesoglcea 
seems  to  be  crowded  out  entirely  in 
the  structure,  and  the  figure  shows 
but  one  cell  belonging  to  this  layer 
and  lying  at  some  distance  from  the 
point  at  which  the  eyes  are  formed. 

.  The  eye  of  a  planarian  worm  fur- 
nishes an  example  of  a  simple  eye  of 
slightly  greater  complexity  than  of 
the  starfish  and  Solen  and  with  more 
highly  specialized  visual  cells  than  in 
the  medusa  eyes  which  have  just 
been  described.  We  shall  study  an 
eye  from  Planaria  torva  and  another  FlG-  204.— Axial  section  of  a  single  eye  of 

e  A  ,.  ,  ,     ,      .        /T-,.  Planaria  torva.     B,  a  similar  section  of 

from    Amandia    polyopthalmia    (Fig. 


204),  and  at  the  same  time  compare 
them  with  the  larger  eye  found  in 
another  planarian  worm,  Planaria 
gonocephala. 

Among  the  many  eyes  distributed 
over  the  dorsal  surface  of  Planaria  torva  are  some  composed  of  a 
single  visual  cell.     As  in  most  planarians,  this  is  sunk  beneath  the 


a  side  eye  of  Amandia  polyopthalmia; 
vis.c.,  visual  cell  nucleus;  pg.c.,  pigment 
cell ;  vis.r.,  sensory  cell-organ  of  light  per- 
ception, corresponding  to  visual  rod;  nv. 
/.,  nerve  fiber  of  visual  cell  extending  to 
central  ganglion.  (After  R.  HESSE  in 
Zeits.f.  wiss.  Zool.) 


232 


HISTOLOGY 


epithelial  surface  from  which  it  originated  and  lies  within  the  body  tis- 
sues, one  of  whose  cells  has  moved  alongside  of  it  and  developed  black 
pigment  in  some  of  its  cytoplasm.  The  pigmented  portion  of  the 
body  is  arranged  in  a  crescentic  form,  with  the  nucleus  placed  on  the 
outer  side  in  a  small  portion  of  undifferentiated  cytoplasm. 

The  distal  cytoplasm  of  the  visual  cell  is  much  enlarged,  and  forms 
a  mass  that  is  bent  laterally  and  lies  in  the  concave  side  of  the  pigment 
cell.  The  distal  edge  of  this  mass  is  modified  into  a  denser  substance, 
the  cell-organ  of  sight  or  rhabdome,  which  forms  a  lining  against  the 
interior  of  the  cup-shaped  pigment  cell. 

It  should  be  noticed  here  that  this  visual  cell  is  inverted;  that  the 
rays  of  light  must  pass  through  the  cell  body  and  nucleus  as  they  enter 
the  opening  of  the  pigment  cup  and  strike  the  rhabdome  from  its  proxi- 
mal side.  The  efferent  pole  of  the  visual  nerve  cell  is  drawn  out  into  a 
fiber  that  passes  inward,  to  some  ganglion  where  its  impulse  can  be 
discharged  and  used. 

Concerning  the  central  connections  of  this  fiber  but  little  is  known. 
It  can  be  said,  however,  that  it  forms  a  very  simple  pathway  for 
the  light-perception  impulse,  the  greater  part  of  the  distance  being 
furnished  by  the  one  drawn-out  process  of  the  visual  cell  itself.  This 
is  not  true  in  any  eye  that  forms  an  image. 

The  eyes  of  other  planarians 
are  often  but  larger  forms  of 
this  monocellular  organ  with  its 
monocellular  pigment  cup.  In 
the  eye  of  Amandia  (Fig.  204, 
B)  there  are  three  or  five  visual 
processes  from  the  one  cell,  with 
the  perceptory  organs  on  the 
ends  of  the  processes.  These 
processes  are  directed  into  the 
same  kind  of  a  mononuclear 
pigment  cup. 

In  Planaria  gonocephala  we 
find  the  same  kind  of  an  organ, 
except  that  there  are  from 
twenty  to  thirty  or  more  visual 
cells,  and  the  pigment  cup  is 
made  of  a  layer  of  over  a  hun- 
dred separate  cells  instead  of 
As  in  the  unicellular  form  of  pigment  cup,  the  nucleus 


FIG.  205.  —  Axial  section  of  the  eye  of  a  planarian 
worm,  Planaria  gonocephala.  vis.c.,  visual  cells ; 
vis.r.,  visual  rods  or  rhabdomes;  nv.f.,  centripetal 
fibers  of  visual  cells;  pg.c.,  pigment  cells.  (After 
R.  HESSE  in  Zeits.f.  wiss.  Zool.) 


only  one  cell. 

is  always  in  the  proximal  part  of  the  cell  body  and  the  pigment  is  in 

the  distal  part  (Fig.  205).    A  slight  difference  is  to  be  seen  also  in  the 


VISUAL    TISSUES  233 

visual  cells.  The  perceptory  organ  is  separated  from  the  cell  body  by 
a  cytoplasmic  process  instead  of  resting  directly  upon  it.  The  afferent 
process  is  thus  almost  a  fiber,  like  the  efferent  process. 

All  these  planarian  eyes  have  sunk  below  the  surface,  and  the  break 
in  the  outer  epithelium  has  been  grown  over  by  its  simple  cubical  cells, 
which  thus  form  a  cornea,  together  with  the  connective- tissue  elements 
between  them  and  the  eye.  But  it  is  an  unspecialized  cornea  and  not 
different  in  any  way  from  the  rest  of  the  integument.  There  is  no  lens 
present. 

Another  eye  type  of  which  space  forbids  a  full  description  is  that  seen 
in  the  leech.  Here  also  we  meet  with  an  organ  that  contains  from  one 
to  upward  of  forty  visual  cells.  The  degree  of  specialization  is  a  high 
unicellular  one.  The  visual  cell-organ  is  peculiar,  and  contains  in  its 
body  both  retinal  and  lens  structures. 

Going  back  to  the  echinoderms,  in  which  we  found  the  simplest  type 
of  eye  in  a  starfish,  we  find  our  next  step  of  development  materialized 
in  one  of  the  urchins,  Diadema,  where  the  eyes  are  developed  on  the 
bases  of  the  spines. 

This  eye  is  somewhat  compound ;  that  is,  it  is  composed  of  a  number 
of  units,  any  one  of  which  would  represent  an  efficient  organ  of  light 
perception.  These  units  are  each  composed  of  several  different  cell 
groups  or  tissues,  one  of  which,  the  superficial  cornea,  is  common  to 
all  of  them.  Each  unit  is  an  upright,  five-  or  six-sided  column  whose 
proximal  end  is  somewhat  pointed  and  rests  in  a  pigment  cup.  This 
pigment  cup  is  composed  of  mesodermal  cells  which  have  moved  up 
from  below  and  formed  the  pit-like  cavity  which  opens  distally.  The 
cup  forms  the  sides  of  the  eye-unit  or  ocellus. 

The  larger  body  of  each  ocellus  is  an  oval  mass,  made  up  of  ten  or 
twelve  large  transparent  and  refractive  cells  with  small  nuclei.  These 
cells  are  elongate  and  roughly  wedge-shaped  with  the  sharp  ends  pointed 
toward  each  other  and  interlocking,  while  the  blunt  ends  rest  on  the  sur- 
face of  the  mass.  This  organ  is  undoubtedly  the  lens,  as  can  be  told 
by  its  transparent  and  refractive  nature  and  the  position  which  it  occu- 
pies. 

The  lens  is  capped  distally  and  proximally  by  two  caps,  each  com- 
posed of  a  single  layer  of  cuboidal  cells  and  each  covering  nearly  a 
third  of  the  lens  surface.  These  caps  are  well  shown  in  two  middle 
ocelli,  where  they  are  represented  as  in  a  surface  view  (Fig.  206).  In 
the  ocelli,  to  right  and  left,  they  are  seen  in  median  section  only.  They 
are  much  alike,  and  only  the  position  seems  to  decide  that  the  proximal 
cap  must  be  composed  of  visual  or  light-perceiving  cells.  The  upper 
cap  has  been  spoken  of  by  the  Saracens  as  a  germinative  group  of  cells 
from  which  the  lens  is  formed  and  removed. 


234 


HISTOLOGY 


As  has  been  indicated,  a  common  covering  of  a  simple  ciliated  epithe- 
lium covers  all  the  ocelli.  It  is  roughly  divided  into  fields  by  the  slight 
curvature  of  the  outer  ends  of  the  ocelli. 

Below,  the  ocellus  is  set  into  the  pigment  cup  with  its  lower  or  visual 
cap  of  cells  lying  in  direct  contact  with  the  pigment  cells.  Nerve  fibers 
come  up  from  the  sub-dermal  nerve  layer  and  provide  the  ocelli  with 
fibrils.  The  eyes  are  evidently  very  efficiently  coordinated  with  the 
nerve  centers,  because,  if  the  light  is  interrupted  from  any  quarter,  the 
animal  at  once  moves  all  its  wicked  spines  so  that  they  point  to  this 
quarter  and  are  ready  to  repel  an  attack. 


nv. 


FIG.  206.  —  Vertical  section  through  an  eye-spot  of  Diadema.  Slightly  schematic,  nv.,  nerve 
layer  which  sends  branches  to  eye  through  the  basal  connective-tissue  layer  (bl.);  p.c., 
proximal  layer  of  sensory  cells.  The  three  eye  units  to  the  left  show  pigment  caps  on  bases. 
Those  on  the  right  are  complete  median  section.  The  two  in  the  middle  are  lateral  surface 
views  to  show  the  outer  and  inner  cell  caps.  (After  C.  F.  and  P.  B.  SARASIN.) 

Although  they  are  far  more  specialized  in  most  ways,  the  Arthropods 
show  a  type  of  eye  that  should  be  studied  at  this  point  on  account  of 
structural  characters  which  lead  one  naturally  to  think  of  them  when 
studying  the  eye  of  Diadema.  This  eye  as  well  as  Diadema's  is  called 
compound  because  it  is  composed  of  a  number  of  similar  units,  each  of 
which  is  apparently  independent  of  the  others  so  far  as  seeing  is  con- 
cerned. When  the  nerve  centers  with  which  this  eye  is  connected  are 
studied,  however,  it  is  apparent  that  the  eye  must  act  as  a  whole  in 
some  manner. 

A  characteristic  feature  of  this  eye  is  the  way  in  which  the  cuticle 
is  carried  into  its  formation.  The  cuticle  seems  to  be  a  more  predomi- 
nant feature  in  this  group  than  in  any  other. 

We  shall  study  the  eye  of  Periplaneta  orientalis  as  a  type  of  the  ar- 
thropod eye,  referring  to  a  crustacean  form,  Palawan  squilla,  for  occa- 
sional comparison.  The  outer  surface  of  these  eyes  shows  a  rather 


VISUAL    TISSUES 


235 


weak  division  into  very  many  (perhaps  10,000)  divisions,  and  a  vertical 
section  through  the  principal  axis  of  the  eye  (Fig.  207)  will  show  that 
each  of  the  superfi- 
cial divisions  repre- 
sents the  base  of  a 
long,  thin  truncated 
cone,  whose  inner 
and  smaller  end 
rests  on  the  small 
semicircular  base- 
ment membrane,  on 
which  the  ends  of 
all  the  other  cones 
also  rest.  Each  of 
these  cones  repre- 
sents a  single  unit 
or  ommatidium  of 
the  eye,  and  it  is 
made  up  of  the  fol- 
lowing kinds  of  cells 
and  cell-products. 

Rising  up  from 
the  basement  mem- 
brane are  a  circu- 
lar group  of  retinula 
cells,  which  are  vis- 
ual nerve  cells.  In 
both  Periplaneta  and 


vis.  n. 


FIG.  207. — Axial  section  of  the  eye  and  eye-stalk  of  Palamon 
squilla.  gn.,  optic  ganglia;  ep.,  epidermis  (hypodermis) ;  vis.n., 
visual  cell  nuclei;  rt.gn.,  retinal  ganglion;  nv.,  optic  nerve. 
(After  SCHNEIDER.) 


Palamon  these  are 
seven    in    number. 

In  Palamon  (Fig.  208)  they  rise  together  equally,  with  their  distal 
ends  enlarged  and  containing  each  a  nucleus.  These  nuclei  are  thus 
at  nearly  the  same  level.  In  Periplaneta  (Fig.  209)  three  of  the  cells 
have  the  larger  mass  of  cytoplasm,  which  is  lower  in  the  general 
cylinder  mass,  and  the  remaining  four  nuclei  are  thus  at  the  top. 
Cell  boundaries  are  hard  to  see  between  these  cells,  and  so  a  longitu- 
dinal section  of  an  ommatidium  of  Periplaneta  appears  to  have  two 
retinula  cells  cut  in  longitudinal  section  and  possessing  each  an  upper 
and  a  lower  nucleus. 

The  inner  edge  of  each  visual  cell  has,  developed  from  its  cytoplasm, 
a  cell-organ  which  has  been  called  a  rhabdomere.  As  all  the  rhabdo- 
meres  fit  closely  together,  they  form  a  single  spindle-shaped  or  club- 
shaped  organ  called  the  rhabdome  (we  shall  hereafter  speak  of  the  single 


HISTOLOGY 


rh. 


FIG.  208.— Part  of  an  ommatidium 

from  the    crustacean,    Palamon  parallel  bristles  Or 
squilla.    rh.,  rhabdomes;  /.,  lens  ,         _,  , 

or  crystalline  cone ;  vis.c.,  visual  rods-       1  nese    rods 

cell  showing   a  nucleus  near  its  come   OUt    at   right 
top.    (After  SCHNEIDER.)  ,  ,, 

angles  to  the  axis 

of  the  cell,  and  are  therefore  at  right  angles  to 
the  light-waves.  They  form  the  plates  spoken 
of  in  the  first  part  of  this  chapter. 

Over  the  expanded  ends  of  the  rhabdomes 
lies  the  end  of  the  lens.  This  is  composed  of 
several  parts,  four  in  number  in  most  arthro- 
pods, and  these  parts  are  formed  by  four  thin 
cells  which  lie  just  outside  of  and  around 
them.  These  are  the  lens-cells  (see  Fig.  209). 
They  rest  proximally  upon  the  retinula  cells 
according  to  Grenacher;  while  according  to 
Patten  they  pass  down  to  the  basement  mem- 


cell-organ,  mentioned  above  as  a  rhabdo- 
mere,  as  a  rhabdome).  This  is  the  only 
part  of  the  whole  eye  that  the  light-waves 
can  affect  so  as  to  produce  a  stimulation, 
and  the  rhabdomes  transmit  this  stimula- 
tion to  the  retinula  cells  as  an  impulse, 
which  is  carried  out  to  the  optic  nerve 
centers  through  the  proximally  produced 
cell  bodies  of  the  retinulae,  or  visual 
cells,  which, 
therefore,  serve  as 
nerve  fibers.  Fine 
nerve  fibrils  have 
been  detected  in 
the  rhabdomes  of 
Palcsmon,  and  they 
pass  through  into 
the  retinula  cell 
(see  Fig.  208).  In 
fact,  it  can  be  seen 
in  Periplaneta  that 
the  whole  rhab- 
dome  is  nothing 
but  an  edge  of  the 
retinula  cell  bear- 
ing a  row  of  in- 
numerable tiny 


FIG.  209. — Longitudinal  section 
of.  a  single  ommatidium  of 
Periplaneta  orientalis.  b.m., 
basement  membrane ;  cu.,  cu- 
ticle divided  into  cornea! 
areas;  rt.c.,  retinula  cells  or 
visual  cells;  rh.,  rhabdome  or 
cell-organ  of  light  perception ; 
/.,  lens;  I.e.,  lens-cells;  c.c., 
corneal  cells;  sup.c.,  support- 
ing cells  (slightly  modified 
hypodermal  cells);  nv.f., 
nerve  fiber.  (After  R.  HESSE 
mZeUs.J.  Wiss.  Zool.) 


VISUAL   TISSUES 


237 


brane  outside  of  them.  Grenadier's  view  certainly  accords  with  the 
facts  as  seen;  while  Patten's  view  seems  to  represent  the  condition 
which  should  obtain  if  both  kinds  of  cells  were  derived  from  the  same 
layer  by  a  simple  process  of  differentiation.  It  is  probable,  however, 
that  the  retinula  and  lens-cells  were  derived  from  the  upper  epithelium 
by  delamination,  through  successive  amitotic  divisions,  as  a  terminal 
process.  This  would  support  Grenacher's  view. 

Lying  still  distad  from  the  crystalline  cone  and  usually  separated 
from  it  by  the  cone  cells  are  four  corneal  cells,  which  are  the  hypodermal 
cells  that  produce  the  cornea.  They  are  flat  and  thin,  and  in  the  insects 
two  of  them  become  pigment  cells  (see  Fig.  209,  c.  c.}.  Other  pigment 
cells  are  found  around  the  ommatidium.  They  are  mesodermal  in 
origin,  and  some  of  them  expand  distally  to  cut  off  an  excess  of  light, 
thus  acting  as  iris  cells. 

There  is  an  almost  infinite  variety  of  arthropod  eyes,  differing  prin- 
cipally in  detail  from  the  two  examples.  This  detail  is  so  extensive, 
however,  that  we 

cannot  begin  to  cu. 

discuss  it.  We 
shall  examine 
two  other  arthro- 
pod eyes,  the 
accessory  eye  or 
ocellus  of  an 
arthropod  and 
the  eyes  of  an 
arachnid,  to  see 
a  simpler  type 
of  visual  organ, 

but  One  which  is  FlG-  2I0.  — Axial  section  through  ocellus  accessory  eye  of  the  larva 
of  a  beetle,  Dytiscus.  cu.,  cuticle;  /.,  lens;  hyp.,  hypodermis;  vis.c., 
visual  cells;  vis.r.,  sensory  or  visual  rods  on  visual  cells;  nv.f., 


nerve  fibers  derived  from  bases  of  visual  cells.     (From  LANG  after 
GRENACHER.) 


yet  capable  of  a 
higher  differen- 
tiation than  the 
extremely  spe- 
cialized insect  and  crustacean  compound  eye.  The  ocellus  of  a  Dytiscus 
larva  shows  a  simple  invagination  of  an  area  of  the  hypodermal  cells 
(Fig.  210).  Those  which  have  left  the  surface  have  lost  all  cuticle-form- 
ing power,  while  those  which  are  still  at  the  surface  and  near  the  closed 
point  of  invagination  have  secreted  an  extra  amount  of  cuticle  as  a  lens. 

The  cells  lying  in  the  fundus  have  developed  visual  organs  in  the 
form  of  small  rods  which  point  distally.  The  proximal  ends  of  these 
cells  are  developed  as  usual  into  efferent  nerve  fibers  (see  Fig.  210). 

These  cells,  which  line  the  sides  of  the  invagination,  meet  across  the 


HISTOLOGY 


line  of  vision,  but  their  distal  ends  are  rendered  transparent  so  that  the 
light  can  pass  through.  They  occupy  a  position  in  which  cells  similarly 
placed  in  other  eyes  would  form  a  lens.  It  is  even  possible  that,  owing 

to  different  indices 
of  refraction  in  dif- 
ferent regions  of  the 
cell  body,  some  sort 
of  lens  function  is 
performed,  but  the 
presence  of  the  cor- 
neal  lens  above  tends 
to  invalidate  such 
a  mere  supposition. 
They  may  be  called 
the  glassy  or  vitreous 
cells. 

In  the  majority 
of  other  insects  the 
retina  layer  of  the 
ocellus  is  derived 
from  the  hypodermis 
in  a  different  man- 
ner. The  first  step 
consists  of  the  invag- 
ination  of  the  hypo- 
dermis, on  the  area 
that  is  to  be  occu- 
pied by  the  ocellus, 
into  a  shallow  pit. 
The  hypodermis  so 
depressed  then  sepa- 
rates into  two  layers,  by  a  proximo-distal  division  of  the  cells  according 
to  some  writers,  more  probably  by  the  recession  of  some  of  the  cells 
of  the  single  layer  into  a  more  proximal  position,  where  they  become 
converted  into  visual  cells  by  the  development  of  rhabdomes  and  the 
extension  of  the  proximal  portion  of  their  bodies  into  efferent  nerve 
fibers. 

The  original  layer  of  hypodermal  cells,  after  the  retinal  cells  have 
been  withdrawn,  becomes  specialized  for  the  transmission  of  light  and 
also  secretes  the  thickened  area  of  the  cuticle,  which  serves  more  or  less 
imperfectly  as  a  lens.  Such  a  layer  of  transparent  cells  may  be  desig- 
nated as  the  vitreous  layer.  It  also  functions  as  the  corneal  layer,  since 
the  lens  which  it  forms  is  also  the  protective  cornea.  These  conditions 


FIG.  211. — Axial  section  of  an  ocellus  of  the  orthopterous  in- 
sect Perla  bicaudata.  cu.,  cuticle  which  is  thickened  at  (/.)  to 
form  the  lens;  hyp.,  hypodermis  which  is  thickened  centrally  to 
secrete  the  lens  and  to  constitute  the  crystalline  body;  vis.c., 
visual  cells  which  were  derived  from  the  modified  central  hypo- 
dermis or  crystalline  layer;  b.m.,  basement  layer  of  connective 
tissue;  nv.,  nerve  composed  of  processes  from  the  visual  cells. 
(After  REDIKORZEW  in  Arch.f.  mik.  Anat.) 


VISUAL    TISSUES 


239 


are  well  demonstrated  by  the  ocellus  of  the  orthopterous  insect,  Perla 
bicaudata,  which  is  represented  in  axial  section  by  Figure  211. 

The  eye  of  Limulus  is  a  simple  form  which  shows  an  approach  to 
the  crustacean  type. 
It  is  formed,  much  as 
was  the  ocellus  of  Dy- 
tiscus,  from  a  surface 
area  of  some  extent 
whose  epithelium  has 
been  invaginated  at 
numerous,  regularly 
placed  points  into 
cup-like  depressions. 
The  lower  surface  of 
the  cuticle,  which  is 
formed  in  Limulus  as 
in  other  Arthropoda, 
all  over  the  surface  of 
the  body,  fits  into 

vis.  c." 
gn.  c. 


ep 


rh.— 


phemus.  Two  of  the  numerous  ocelli  are  shown,  one  in  me- 
dian axial  section  (^4);  the  other  by  a  lateral  surface  view 
(B).  cu.,  cuticle,  which  forms  a  partial  lens  over  each  ocellus; 
ep.,  epidermis  which  secretes  the  cuticle  and  from  some  of  whose 
cells  the  visual  cells  (vis.c.)  and  the  ganglion  cells  (gn.c.)  were 
formed;  rh.,  rhabdome;  nv.f.,  nerve  fiber.  (After  WATASE.) 


these  depressions, 
while  the  upper  sur- 
face is  even  and  con- 
tinuous and  shows 

Scarcely  any  effects  of    FlG   2I2.  _part  of  an  axial  section  of  the  eye  of  Limulus  poly- 

the  invaginations  be- 
neath. It  thus  forms 
a  lens. 

In  the  bottom  of 
each  epithelial  cup  a 
group  of  some  fifteen  to  twenty  cells  of  the  layer  are  considerably 
enlarged  and  grouped  together  to  form  a  melon-shaped  body,  which 
is  the  retinal  portion  of  the  organ.  The  cells  are  now  differentiated 
into  visual  nerve  cells  or  retinulae  by  the  development  of  a  lateral 
rhabdome  edge  and  production  of  the  proximal  cell  body'  into  an 
efferent  nerve  fiber  (Fig.  212). 

It  is  in  the  spider  that  one  of  the  most  interesting  visual  conditions 
is  found.  This  animal  possesses  many  eyes  on  its  dorsal  surface,  and 
both  direct  and  inverted  eyes  can  be  found  among  them.  Figure  213 
shows  a  picture  of  a  section  taken  vertically  through  two  of  the  eyes 
of  Epeira  diadema.  To  the  left  is  a  direct  eye  called  the  principal  eye, 
while  to  the  right  is  the  accessory  eye,  which  is  inverted.  Each  of  these 
eyes  was  derived  from  an  epidermal  invagination  which,  later,  was 
differentiated  from  the  other.  Both  of  these  eyes  consist  of  a  simple 


240 


HISTOLOGY 


invagination  of  the  surface  epithelium  which  was  then  cut  off  and  the 
edges  of  the  outer  layer  made  continuous  again.  This  layer  appears 
in  the  figure  (213,  A)  as  the  corneal  layer,  and  is  responsible  for  the 
formation  of  the  cuticle,  which  is  enlarged  at  the  point  above  each  eye 
into  the  cuticular  lens. 

The  method  of  development  of  visual  cells  on  the  walls  of  the  invagi- 
nation is  puzzling,  and  the  accounts  of  various  investigators  are  not 
entirely  satisfactory.  The  explanation  in  the  case  of  the  accessory  eye 
with  its  inverted  visual  cells  is  evidently  correct.  The  epithelium  on 
the  upper  or  distal  wall  of  the  invagination  becomes  the  visual  cell-layer. 
Thus  the  distal  end  of  the  visual  cell  forms  the  rhabdome,  and  the  proxi- 


vis, 


FIG.  213.  —  Anterior  and  posterior  eyes  of  the  spider,  Epeira  diadema.  I.,  cuticular  lens;  hy., 
hypodermal  layer;  rt.c.,  retinula  cells;  vis.r.,  visual  rods  or  rhabdomes;  nv.f.,  nerve  fibers. 
(Alter  GRENACHER.) 

mal  end  (now  directed  outward)  is  elongated  to  constitute  the  efferent 
nerve  fiber.  The  other,  or  inner,  wall  of  the  invagination  becomes  thin 
and  is  transformed  inlo  a  tapetum. 

In  the  principal  eye  we  have  a  startling  exception  to  the  usual  rule 
that  the  rhabdome  is  distal  and  the  nerve  fiber  proximal  in  an  epithe- 
lium specialized  visually.  Also  we  have  a  completely  lost  layer  to 
account  for  if  the  eye  were  formed  by  invagination  instead  of  by  delami- 
nation.  This  eye,  like  the  other,  is  described  as  arising  by  a  posteriorly 
directed,  flat  invagination.  Its  visual  epithelium  is  also  developed  on 
the  outer  wall  of  this  cavity.  But  the  rhabdome  appears  in  the  proximal 
part  of  the  cell,  and  the  nerve  processes  not  only  are  twisted  around  so 
as  to  leave  the  distal  end  of  the  cell,  but  they  thereby  are  obliged  to  pene- 
trate the  lumen  (which  is  here  closed  to  a  plane)  and  to  pass  through 
the  opposite  wall,  which  is  finally  lost  without  leaving  a  trace  in  the 
adult  structure. 


VISUAL    TISSUES 


24I 


\ 


vta.  c. 


The  eyes  of  all  higher  creatures  are  organized  to  form  a  single  image, 
and  it  can  be  demonstrated  that  an  image  is  formed  by  most  of  them. 
An  owl's  or  other  bird's  or  mammal's  eye,  when  the  posterior  side  is 
cleaned  of  fat,  muscles,  etc.,  and  the  corneal  surface  is  pointed  at  a  bright 
window,  shows  a  beautifully  distinct  picture  of  the  window  on  its  retina 
from  the  rear.  We  also  know  that  this  image  is  transmitted  to  the 
brain  and  consciousness  in  the  case  of  our  own  eye.  The  question  arises 
as  to  how  much  of  an  image  is  formed  by  the  compound  eye.  It  has 
been  concluded  that  no  recognizable  image  is  produced  in  some  cases; 
while  to  see  a  male  Papillio  dart  at  a  colored  paper  representation  of 
its  mate,  convinces  one  that  this  insect,  at  least,  can  see  and  recognize 
form.  With  many  other  insects  the  eye  probably  does  not  record  a 
form  or  the  brain 
does  not  remember 
it,  but  it  is  very  quick  vis. 
to  perceive  any  rela- 
tive motion  in  the 
objects  around  it. 

One  more  com- 
pound eye  with  sepa- 
rate lenses  may  be 
studied  in  a  serpulid 
worm,  Branchiomma. 
Figure  214  represents 
this  eye  a  trifle  dia- 
grammatically.  The 
visual  cells  have  de- 
veloped out  of  every 
third  epithelial  cell, 
in  section,  and  the 
intervening  cells  are 
pigment  (iris)  cells. 
The  visual  cell  of  this  eye  is  unusual  in  that  it  secretes  a  lens  in  its 
distal  cytoplasm  as  well  as  forming  a  nerve  and  visual  cell-organ. 
Besides  doing  all  this  it  forms  or  takes  part  in  forming  the  cuticle.  These 
various  functions  are  the  separate  duties  of  differentiated  cells  in  higher 
forms,  as  the  arthropods. 

The  more  complete  yet  simpler  eyes,  with  single  lens  and  a  retina 
which  receives  an  image,  are  found  among  other  worms  and  mollusks. 
One  has  already  been  alluded  to,  but  not  described,  in  a  medusa. 

No  worm  eye  is  to  be  found  that  is  as  low  in  its  organization  as  that 
of  many  mollusk  forms  such  as  Solen,  etc.  On  the  other  hand,  the 
mollusks  have  the  greatest  variety,  some  of  which  are  among  the  highest 


hyp.  c. 


FIG.  214. — Eye  from  the  serpulid  worm,  Branchiomma.  cu., 
cuticle;  hyp.c.,  hypodermal  cells;  sup.c.,  supporting  cells 
(slightly  modified  hypodermal  cells) ;  vis.c.,  visual  cells  (highly 
modified  hypodermal  cells);  vis.r.,  visual  rods  appearing  as 
curved  hairs;  /.,  lens  formed  by  visual  cells.  (After  HALLER.) 


242 


HISTOLOGY 


and   most   efficient  eyes    known,  and  far  better  organized  than  any 
worm's. 

The  forms  of  Nereis  present  a  visual  organ  which  well  represents 
the  average  type  of  worm  eye.  Figure  215  represents  the  eye  of  a  small 
pelagic  Nereis  of  unknown  species.  This  eye  is  plainly  an  invagination 
of  the  epidermal  layer,  and  has  been  subsequently  constricted  off  from 
the  hypodermis  at  the  point  of  invagination.  The  epidermal  layer, 
after  the  constriction  closes  over,  becomes  the  cornea  with  its  cuticle. 


O" 


FIG.  215. — A,  eye  of  a  pelagic  Nereis,  cu.,  cuticle;  hyp.,  hypodermis  (a  simple  epithelium 
with  very  long  cells) ;  ret.,  retina,  developed  by  the  specialization  of  an  invaginated  area  of 
hypodermis;  /.,  lens;  bl.s.,  blood  sinus.  X  200.  B,  small  portion  of  retina  much  enlarged. 
vis.c.,  visual  cells;  pg.c.,  pigment  cells  which  surround  the  lower  two  thirds  of  the  visual 
cells;  vis.r.,  visual  rods.  X  870.  C,  slightly  oblique  horizontal  section  across  the  retina 
near  level  of  tops  of  pigment  cells,  x  870. 

The  sac-like  eye  changes  its  alternate,  epithelial  lining  cells  into  visual 
cells  and  pigment  cells.  This  takes  place  in  a  greater  degree  on  the 
posterior,  inside  surface  of  the  sac,  and  the  visual  specialization  becomes 
less  and  less  toward  the  anterior  side,  where  for  a  short  space  the  cells 
are  clear  and  transparent  to  permit  of  the  entrance  of  light-rays.  The 
pigment  cells  of  this  eye  form  a  sheet  when  seen  from  the  surface  (Fig. 
215,  C),  and  spaces  in  this  wall  denote  the  presence  of  the  visual  cells 
with  their  rhabdomes  or  visual  cell-organs.  This  rhabdome  consists 
of  a  heavy  cylindrical  bar  of  some  length,  in  the  center  of  which  is  an 
axial  filament. 


VISUAL    TISSUES 


243 


The  lens,  as  in  all  worms,  is  an  extra-cellular  material  secreted  largely 
by  the  upper  epithelium  of  the  sac.  In  many  other  worms  the  lens  is 
secreted  by  a  single  special  cell  which  lies  just  outside  the  retina  and 
passes  the  secretion  into  the  optic  vesicle  through  a  cleft  in  the  sensory 
epithelium.  This  occurs  on  the  extreme  inner  wall,  in  the  worm  Phyl- 
lodoce  ganimosa,  the  imagination  of  whose  eye  is  not  entirely  completed. 
It  also  is  seen,  in  a  somewhat  peculiar  form,  in  the  eye  of  Vanadisformosa 
as  described  by  Hesse.  We  shall  outline  the  structure  of  this  eye  as 
a  most  highly  specialized  type  of  worm  eye  (Fig.  216). 


vit. 


FIG.  216.  —  Eye  of  the  worm,  Vanadis  formosa.  I.,  lens;  vit.,  vitreous  body;  vit.c.,  large  vit- 
reous cell  which  forms  the  vitreous  body;  ret.,  retina  with  narrow  layer  of  pigment  cells ; 
nv.c.,  nerve  cells  forming  ganglia.  (From  HALLER  after  HESSE.) 


The  original  invagination  was  the  same  as  in  Nereis,  and  the  subsequent 
formation  of  a  cornea  by  the  surface  was  also  the  same,  excepting  that  a 
small  amount  of  connective  tissue  crept  in  between  the  cornea  and  the 
optic  sac,  as  we  shall  call  the  invagination.  The  difference  comes  in 
the  larger  number  and  greater  complexity  of  the  structures  formed  by 
the  sac,  and,  what  is  still  more  significant,  the  addition  of  nervous  ele- 
ments to  the  rear  of  the  retina,  probably  to  correlate  the  image  elements 
and  prepare  them  for  their  reception  in  the  central  ganglia. 

The  retina  consists,  as  in  the  other  worms,  of  the  specialized  cells  on 
the  proximal  side  of  the  optic  sac.  Unlike  Nereis,  however,  this  special- 
ization is  confined  to  a  sharply  marked  area,  and  the  visual  cells  are  far 
more  numerous  and  larger.  The  pigment  cells  are  reduced  to  a  narrow 


244 


HISTOLOGY 


band  lying  at  the  level  of  the  visual  cells,  at  the  other  point  where  these 
give  off  the  visual  rods. 

The  epithelium  lining  the  anterior  part  of  the  sac  is  differentiated  into 
two  regions.  That  on  the  extreme  front  has  secreted  the  round  lens,  a 
cell-product.  A  narrow  band  of  the  same  cells,  lying  between  the  retina 
and  the  lens-forming  area,  produce  the  vitreous  body,  which  probably 
further  arranges  the  light-rays  for  their  reception  by  the  retina.  One  of 


bl 


FIG.  217.  —  Eye  of  Strombus  gigas.     I.,  lens;  ret.,  retina;   vit.,   vitreous   body   (fluid);    bl.sp., 
blood  spaces;  bl.,  blood;  nv.,  nerve;  cor.,  cornea,     x  475. 

these  vitreous  body  gland  cells  is  far  larger  than  the  others,  and  has 
retired  outside  the  body  of  the  eye-sac  and  lies  in  a  lateral  position. 

The  lowest  mollusk  eyes  have  already  been  described.  There  are  an 
infinite  number  which  are  graded  from  the  form  possessed  by  Solen  up 
to  forms  that  compare  in  their  degree  of  specialization  with  the  eye  of 
Nereis  among  the  worms.  In  Patella  rota  we  find  a  pit  such  as  the  eye 
of  Solen  would  form  if  half  invaginated.  Arochus  magnus  shows  an 
eye  which  would  represent  that  of  Patella  were  the  invagination  to  be 
almost  completed  and  the  lens  material  formed  into  a  round,  well-shaped 
ball.  In  Turbo  cceniformis  the  structure  is  a  complete  invagination,  and 
a  cornea  is  for  the  first  time  formed  by  the  cutting  off  of  the  invaginated 


VISUAL    TISSUES 


245 


sac  and  the  closing  of  the  epidermal  layers.  This  condition  can  be  seen 
in  Figure  217,  representing  the  eye  of  the  common  Florida  conch, 
Strombus  gigas. 

This  beautiful  eye  is  well  developed  in  all  particulars,  although  not  so 
complex  as  that  of  the  cephalopods.  The  eye-sac  invagination  has  been 
cut  off  from  a  well-developed  epithelial  cornea  to  which  a  thick  layer  of 
connective  tissue  has  been  added.  The  sac  ...-,.,..,...., 

lies,  almost  free,  in  a  large  blood  space  in 
which  blood  coagulum  and  blood  cells  can  be 
seen.  The  sac  is  connected  with  the  sur- 
rounding tissues  by  a  few  plate-like  strands 
of  connective  tissues.  The  blood  space  is 
supplied  with  the  blood  by  a  vessel  which  can 
be  traced,  together  with  the  nerve,  through 
the  entire  length  of  the  long  eye-stalk. 

The  lining  epithelium  of  the  eye-sac  is 
developed  into  the  visual  cells,  the  supporting 
elements  (and  several  other  cells).  This  is 
done  in  a  far  greater  degree  on  the  posterior 
side  of  the  sac,  where  these  cells  form  the 
retina.  This  retina  diminishes  in  thickness 
as  it  is  examined  more  toward  the  front, 
until  first  the  rods  suddenly  terminate  and 
later  the  pigment  is  lost,  likewise  abruptly, 
and  then  there  is  left  only  a  very  thin  layer  of 
simple  epithelium  lying  on  its  outer  basal 
membrane  and  forming  a  transparent  layer 
to  let  the  light-rays  pass  in. 

This  anterior  region  of  the  eye-sac  is  in 
direct  contact  with  the  lens,  a  spherical  body 
with  a  denser  outer  shell.  The  lens  is  a  non- 
cellular  structure  probably  formed  by  the  an- 
terior cells  of  the  sac  which  touch  it.  It  is 
not  as  large  as  the  inside  of  the  cavity  of  the 
sac,  but  there  remains  between  it  and  the  retina 
a  crescentic  (in  section)  space  filled  with  the 
vitreous  fluid.  In  two  other  worms,  it  will  be 
remembered,  this  fluid  was  secreted  by  spe- 
cial cells.  This  does  not  hold  in  the  conch, 
for  the  fluid  is  secreted  from  the  entire  sur- 
face of  the  retina,  as  we  shall  presently  show. 

The  visual  cells  form  the  largest  part  of  this  retina,  which  is  wonder- 
fully clear  and  easy  to  study  (Fig.  218).  They  extend  from  the  nerve 


FIG.  21.8.  —  Enlarged  portion  of 
retina  of  Strombus  gigas.  vis.c., 
visual  cell  whose  yjsual  rod 
(vis.rd.)  projects  above  from 
the  distal  surface  of  the  pig- 
ment layer;  ac.c.,  nucleus  of 
accessory  cell  which  makes  and 
operates  the  pigment  and  also 
forms  the  vitreous  fluid ;  vit.fl., 
vitreous  fluid  which  emerges 
in  thin  streams  from  between 
the  visual  rods;  nv.f.,  nerve 
fiber  bundles.  X  920. 


246  HISTOLOGY 

bundles  near  the  outer  basement  membrane  to  a  point  one  half  the 
thickness  of  the  retina,  where  they  stop  and  are  continued  distally  as  the 
visual  rods.  Between  the  visual  cells  are  found  the  sustentacular  cells, 
which  extend  as  thin  fibrillar  bodies  from  the  basal  membrane  up  to  the 
tops  of  the  visual  cells.  Here  they  expand  into  a  pigmented  cyto- 
plasm, which  surrounds  and  lies  between  the  ends  of  the  visual  cells. 
The  nuclei  of  the  visual  cells  are  round  and  full,  while  those  of  the  susten- 
tacular cells  are  very  much  smaller  and  elongated. 

The  visual  rods  are  nearly  as  wide  and  as  long  as  the  cells  from  which 
they  come.  Passing  down  their  center  almost  to  the  tip  is  an  axial 
filament,  which  is  of  considerable  thickness  and  is  fibrillar.  The  ends  of 
the  rods  are  frequently  shrunken  by  the  process  of  preparation,  but, 
when  seen  to  advantage,  they  are  full  and  rounded  and  almost  touch  the 
vitreous  body.  The  material  of  this  body  extends,  as  very  regularly 
arranged  threads,  up  between  the  rods  as  far  as  the  pigment  layer,  where 
it  is  lost.  The  writers  believe  that  these  threads  represent  a  flow  of 
secretion  from  some  cells  in  the  retina  to  supply  the  growing  vitreous 
body.  Just  which  cells  produce  this  secretion  was  not  determined. 
Judging  from  what  is  known  of  other  worms,  it  must  be  the  pigment 
cells. 

The  nerve  fibers  form  series  of  bundles  lying  near  the  basal  mem- 
brane. They  are  seen  cut  in  transection  in  Figure  218.  Also  one  of  the 
visual  cells  may  be  seen  sending  a  process  into  the  bundles.  The  basal 
membrane  is  very  well  marked  both  in  form  and  staining  power.  It  is 
lined  externally  by  thin  flat  cells,  one  of  whose  nuclei  is  to  be  seen  in  the 
figure. 

In  all  the  lower  mollusk  forms  the  sensory  cells  possess  much  the 
same  kind  of  visual  rod  or  rhabdome.  The  rhabdome  is  varied  to  a 
rather  more  elaborate  and  fan-shaped  rod  in  Helix. 

There  remain  yet  two  very  complex  mollusk  eyes  to  be  described, 
that  which  is  found  on  the  mantle  edge  or  back  of  many  plecypod  mol- 
lusks  as  Pecten,  Spondylus,  and  Oncidium,  and  that  of  the  dibranchiate 
Cephalopods.  These  are  most  complex  in  structure  and  probably  the 
most  efficient  of  the  eyes  of  invertebrate  animals. 

The  eyes  of  Pecten  are  found  scattered  on  the  edge  of  its  mantle  folds 
on  short  stalks,  or  lying  directly  on  the  surface.  Among  the  former 
is  the  eye  of  Pecten  irradians,  a  common  American  form  found  on  the 
eastern  coast  of  the  United  States. 

The  entire  eye  is  covered  with  a  simple  epithelium  bearing  a  very 
insignificant  cuticle  and  resting  on  a  well-developed  basement  mem- 
brane. Most  of  these  cells  are  tall  and  are  heavily  pigmented  in  their 
proximal  ends,  the  nucleus  lying  midway  in  the  cell  at  the  point  where 
the  pigment  ceases.  The  area  forms  a  very  broad  band  about  the  equa- 


VISUAL    TISSUES 


247 


torial  region,  leaving  a  somewhat  thinner  and  very  transparent  pole  of 
the  eye  for  the  light-rays  to  come  through.  In  the  corneal  cells  the 
nucleus  is  placed  somewhat  higher. 

The  place  inside  of  this  outer  covering  contains  the  remaining  organs 
of  the  eye  in  the  following  order  from  distal  to  proximal  positions.  A 
lens,  a  blood  sinus,  the  several  layers  of  the  retina,  a  space  filled  with  a 
vitreous  humor,  the  argentea,  and  the  pigmented  tapetum.  These  all 
lie  in  an  oval  space,  the  eye-sac,  and  a  nerve  approaches  this  sac  through 


FlG.  219. — Eye  of  Pecten  irradians.  cor.,  cornea;  /.,  lens;  bl.s.,  blood  sinus;  o.gn.c.,  outer 
ganglion  cells  upon  which  lies  a  layer  of  supplying  nerve  fibers;  vis.c.,  visual  cells;  lim.m., 
limiting  membrane;  vis.r.,  visual  rods;  ar.,  argentea;  tap.,  tapetum;  nv.f.,  nerve  fiber. 
(After  PATTEN.) 

the  proximal  connective  tissue  and  divides  into  two  branches,  a  basal 
branch  and  a  lateral  branch. 

The  lens  is  unlike  so  many  of  the  lenses  we  have  previously  examined, 
in  that  it  is  composed  of  a  mass  of  cell  bodies  instead  of  secreted  cell- 
products.  These  cells  have  round  nuclei  of  moderate  size,  and  are  nearly 
fitted  together  to  form  a  round  body  with  curved  surfaces  coming  to  a 
sharp,  circular  edge.  This  body  lies  directly  against  the  cornea  whose 
proximal  surface  is  in  contact  with  its  distal  surface,  and  its  edges  reach 
as  far  as  the  pigment  area  of  the  cornea  or  iris.  Its  proximal  curve, 
projecting  into  the  blood  space,  almost  touches  the  next  organ,  which  is 
the  retina  (Fig.  219). 

This  retina  is  a  thick  mass  of  tissue  inclosed  in  a  separate  membrane 


248 


HISTOLOGY 


and  reaching  across  the  eye-sac,  so  that  it  separates  the  blood  sinus  in 
front  from  the  vitreous  fluid  behind.  Its  most  important  layer  is  the 
layer  of  visual  cells,  and  these  are  found  on  its  proximal  surface  pointing 
backward.  This  makes  the  eye  an  inverted  form  like  that  of  the  spider 
and  that  of  the  scorpion.  As  we  shall  see  later,  this  peculiarity  is  also  true 
of  the  human  eye. 

The  visual  cells  form  a  thick  layer  and  are  rather  peculiarly  arranged. 
Their  distal  ends  give  off  the  long,  heavy  visual  rods,  each  of  which  con- 
tains an  axial  filament.  Hesse  describes  two  filaments  in  an  occasional 
rod  of  Pecten  tigrinus.  These  rods  form  a  very  even  and  thick  layer  in  the 
section.  The  line  at  which  they  all  take  their  origin  from  the  visual  cells 
is  straight  and  even,  and  marked  by  a  set  of  fine  plates  in  the  substance 
between  the  cells.  These  plates  are  parts  of  a  large,  continuous  mem- 
brane, the  limiting  membrane,  which  has  many  openings  for  the  rods  to 
pass  through.  Miss  Hyde  has  described  these  rods  as  separate  cells 
beginning  at  and  separated  from  the  visual  cells  by  the  limiting  mem- 
brane, and  each  with  a  nucleus  of  its  own.  Hesse  does  not  find  this 
nucleus  in  several  other  species  of  Pecten,  and  in  Spondylus,  and  the 
writers  could  not  find  it  in  Pecten  tenuicostus. 

The  visual  cells  come  from  the  sides  of  the  retina,  and  by  well-grad- 
uated curves  turn  in  a  proximal  course  until  they  end  on  the  limiting 
membrane,  where  the  rods  are  given  off.  Their  nuclei  are  large  and  oval, 

and  lying  among  them  are 
a  number  of  slender  susten- 
tacular  cells  with  long,  thin 
nuclei.  These  supporting 
elements  give  off  fibers 
which  seem  to  pass  out- 
ward and  reach  the  layer  of 
the  cells  above  them. 

Lying  still  outside  the 
layer  of  visual  cells  is  a 
single  layer  of  stout,  heavy 
cells  with  large  nuclei.  They 
are  called  the  outer  gan- 
glion cells.  This  layer  shows 


Urn.  m. 


FIG.  220. — Two  outer  ganglion  cells  and  three  visual 
cells  from  the  retina  of  Pecten  irradians  to  show  their 
connection  through  nerve  fibrils,  nv.fi.  These  fibrils 
form  a  spiral  coil  around  the  nuclei  of  the  outer  gan- 
glion cells,  and  pass  into  the  main  visual  cells,  lim.m., 
limiting  membrane.  (Modified  from  Miss  HYDE.) 


a  slight  differentiation  in 
ordinary  preparations,  and 
under  proper  methylene- 
blue  treatment  it  is  seen 
that  the  row  as  seen  in 
section  is  composed  of  alternate  nerve  and  supporting  cells.  The 
methylene  blue  shows  that  neuro-fibrils  enter  this  layer  from  the  lateral 


VISUAL    TISSUES  249 

nerve  branch,  and  that  these  or  others  wind  in  a  spiral  through  the  cyto- 
plasm around  the  nucleus  of  these  nerve  cells,  and  then  pass  out  inwardly 
to  send  branching  ends  to  the  visual  cells  (Fig.  220).  Miss  Hyde  does 
not  show  an  outer  (one  cannot  say  as  yet  "proximal"  or  "distal")  area 
of  cytoplasm  on  these  external  ganglion  cells,  as  Hesse  and  others  have 
pictured,  in  an  analogous  position,  in  so  many  other  pecten  eyes  and  as 
the  writers  saw  in  Pecten  tenuicostus.  Some  faint  indication  of  it  is 
shown  in  one  of  her  figures. 

This  edge  shows  that  the  cells  of  this  layer  have  some  very  peculiar 
nervous  function  besides  their  connection  with  the  visual  cells  as  de- 
scribed by  Miss  Hyde  and  as  weakly  shown  by  certain  fibrils  in  the  figures 
of  Patten  and  Hesse.  The  edge  is  drawn  out  into  thick  groups  of  rod- 
like  processes,  which  converge  toward  an  outside  central  point  and  join 
the  upper  branches  of  the  optic  nerve.  In  doing  this,  it  can  be  seen, 
they  come  to  act  as  intermediate  cells  between  the  visual  cells  and  the 
central  ganglion.  These  visual  cells  also  have  a  direct  connection  by 
means  of  the  lower  or  lateral  branches  of  this  nerve,  so  that  there  are  two 
different  pathways  for  the  impulse,  and  we  have  a  case  where  communi- 
catory nerve  cells  have  entered  the  retina  as  they  had  begun  in  the  worm, 
Vanadis,  where  they  formed  a  ganglion  below  the  retina  and  outside  of  the 
eye.  This  layer  of  cells  probably  has  some  function  to  perform  which  is 
analogous  to  that  of  the  layers  of  ganglion  cells  in  the  eye-stalk  of  the 
Arthropoda  or  the  ganglion  layers  of  the  vertebrate  retina. 

Beneath  the  retina,  and  separated  from  it  by  the  wide  space  filled 
with  the  vitreous  fluid,  is  the  tapelum,  a  layer  filled  with  refracting 
granules  and  used  apparently  to  reflect  the  light.  This  layer  is  formed 
by  a  single,  wide,  thin  cell  in  other  pectens,  and  the  same  will  probably 
be  found  true  in  the  form  we  are  studying.  Underneath  the  tapetum  is 
the  pigment  layer,  a  simple  epithelial  covering  whose  thick  cubical  cells 
are  filled  with  red  pigment  granules. 

The  dibranchiate  cephalopod  eye  is  probably  one  of  the  most  complex 
in  existence,  although  this  complexity  is  more  easily  understood  and  more 
superficial  than  in  the  vertebrate  eye.  The  eyes  of  both  of  these  groups 
should  be  studied  from  the  developmental  point  of  view  to  properly 
understand  their  histology. 

The  eye  of  the  squid,  Loligo  Pealii,  begins  as  an  invagination  which, 
as  in  so  many  other  mollusks,  becomes  the  visual  sac  (Fig.  221).  At 
an  early  period  this  sac  becomes  cut  off  from  the  exterior,  and  develops 
the  posterior  part  of  its  lining  epithelium  into  the  visual  epithelium. 
The  anterior  region  develops  on  its  inner  surface  a  rather  larger  part  than 
one  half  of  the  lens.  This  lens  is  a  homogeneous  cell-product  which  is 
formed  as  a  series  of  very  perfect  lamellae  lying  parallel  to  each  other. 
The  ectodermal  surface,  which  faces  outward  from  this  inner  lens  epi- 


250 


HISTOLOGY 


thelium,  forms  the  other  part  of  the  lens  as  a  distally  directed  and  supple- 
mentary portion  of  the  whole  outline  of  the  lens,  which  is  almost  spherical. 
Thus  the  surface  epithelium,  which  in  other  mollusks,  as  Strombus,  is 
used  to  form  the  cornea,  in  this  case  forms  the  outer  part  of  the  lens; 
while  the  inner  surface,  which  forms  the  entire  lens  of  Strombus  and 
other  higher  mollusks,  in  Loligo  forms  the  proximal  two  thirds  of  the 
lens. 

It  will  be  remembered  how,  in  most  of  the  previously  described 
mollusks'  eyes,  an  area  of  pigment  was  placed  about  the  cornea  to  keep 


FIG.  221.  —  Five  outline  sketches  to  illustrate  the  histogenesis  of  the  eye  in  a  dibranchiate 
cephalopod.  ret.,  retinal  epithelium;  /./.,  fold  which  forms  the  lens;  /.,  lens;  ir.t  iris  fold; 
cor.,  corneal  fold.  (After  LANG.) 

too  much  light  or  too  oblique  rays  of  light  from  entering  the  eye-sac. 
This  same  region  of  pigmented  epithelium  is  found  in  Loligo,  but  it  has 
been  raised  into  a  circular  ridge,  and  then  this  ridge  is  drawn  centrally 
as  a  circular  septum,  with  a  central  aperture  for  the  light  to  pass  through. 
The  amount  of  light  to  be  admitted  is  thus  determined  by  the  septum, 
which  can  enlarge  or  diminish,  the  central  opening.  It  thus  becomes  a 
very  perfect  form  of  iris. 

Still  a  third  ridge  of  integument,  lying  outside  of  the  iris,  is  now 
developed  and  closed  in  centrally  until  it  forms  an  external  covering  for 
all  the  other  structures.  This  is  the  cornea  which,  it  can  be  seen,  is  not 
homologous  with  that  of  any  other  mollusk.  The  tissues  over  the  eye 
are  transparent,  and  are  composed  of  an  outer  and  inner  epithelium  with 


VISUAL    TISSUES 


251 


a  small  amount  of  connective  tissue  between.  The  corneal  fold  never 
entirely  closes  up  in  many  cephalopods,  but  leaves  a  minute  water  canal. 
Thus  the  corneal  cavity  is  filled  with  sea  water,  to  which  is  possibly  added 
some  secretion  of  the  neighboring  cells  to  keep  this  water  free  from  para- 
sites. 

A  few  histological  details  should  be  explained  before  we  proceed  to 
the  more  important  retina.  The  tissues  back  of  the  primary  eye-sac 
become  much  specialized.  They  are  mostly  of  various  connective-tissue 


FIG.  222. —  A,  B,  C.  Three  stages  in  the  histogenesis  of  the  retina  in  the  cephalopod  mol- 
lusks  (A,  B,  from  Sepia;  C,  from  Eledone);  b.m.,  basement  membrane,  below  which  the 
nucleated  bodies  of  the  visual  cells  have  migrated  in  B,  C  ;  p.lim.m.,  proximal  limiting 
membrane;  d.lim.m.,  distal  limiting  membrane;  vis.c.,  visual  cells;  ac.ret.c.,  accessory  reti- 
nal cells;  c.t.c.,  connective-tissue  cells,  one  of  which  contains  an  intra-cellular  blood  channel; 
pg.,  pigment  in  the  distal  cytoplasm  of  the  accessory  retinal  cells;  vis.r.,  visual  rods.  (After 
HESSE.)  X  600. 

forms,  and  some  develop  into  smooth  muscles  to  move  the  eye  1p  a  small 
degree.  A  cartilaginous  capsule  is  formed  so  that  it  incloses  most  of  the 
eye-sac,  and  is  provided  with  many  perforations  through  which  the 
nerve  fibers  pass  from  the  retina  to  the  huge  optic  lobe  of  the  brain. 
This  lobe  is  usually  in  very  close  contact  with  the  eye-sac.  The  double 
layer  of  epithelium  which  forms  the  two  parts  of  the  lens  is  an  interesting 
study,  for  the  details  of  which  we  have  no  space ;  also  the  rigid  connec- 
tive-tissue elements  on  the  outside  of  the  eye-sac  and  iris. 

The  retina  is  naturally  the  most  important  tissue.     It  begins,  as  in 
other  mollusks,  as  a  columnar  epithelium,  whose  elements  lengthen  until 


HISTOLOGY 


FIG.  223.  — .4,  bit  of  retina  from  the  eye  of  a  12  mm. 
squid,  Loligo  Pealii;  p.lim.m.,  proximal  limiting 
membrane,-  b.m.,  basal  membrane  (now  situated 
above  the  real  base  at  b.) ;  vis.c.,  visual  cell  bodies ; 
acc.c.,  accessory  cell  bodies;  vis.r,,  visual  rods 
showing  the  pigment  ascending  halfway,  about 
the  central  axis;  nv.f.,  nerve  fibers  forming  basal 
layer;  mil.,  mitosis  in  accessory  cell;  B,  visual 
rod  from  a  visual  cell  of  a  half-grown  Octopus 
Americanum.  The  animal  was  out  in  full  tropi- 
cal sunlight  for  an  hour  or  two  before  being 
thrown  into  a  strong  fixative,  and  the  pigment 
has  ascended  around  the  central  axis  and  col- 
lected as  a  knob  at  the  distal  end.  X  920. 


they  appear,  as  in  Figure  222, 
A,  resting  proximally  on  a  basal 
membrane.  At  this  time  some 
of  them  begin  to  form  a  layer 
of  visual  rods  on  their  distal 
ends.  The  rod  layer  is  marked 
off  from  the  cells  by  a  slight 
membrane,  the  limiting  mem- 
brane. 

Figure  222,  B,  shows  the 
next  stage  in  development, 
which  consists  in  a  proximal 
migration  through  the  basal 
membrane  of  all  the  rod-bear- 
ing or  visual  cells,  leaving  a 
single  row  of  their  neighbors 
distal  of  this  boundary  (Fig. 
222,  C).  These  latter  cells  do 
not  function  as  supporting  ele- 
ments, but  are  used  to  produce 
and  operate  two  materials:  a 
brown  pigment  to  be  used  as  a 
protecting  pigment,  and  a  fluid 
which  is  conducted  distally  be- 
tween the  rods  and  forms  the 
outer  or  distal  limiting  mem- 
brane. 

This  membrane  is  of  con- 
siderably greater  thickness  in 
some  forms  than  in  others,  and 
is  exactly  homologous  with  the 
fluid,  vitreous  body  which  was 
seen  in  the  eye  of  Strombus. 
This  latter,  too,  was  doubtless 
formed  by  the  sustentacular 
cells  found  in  the  retina,  and 
was  delivered  into  the  eye-sac 
lumen  as  the  threads  we  saw 
passing  distally  between  the 
visual  rods.  These  cells,  which 
we  shall  call  accessory  retinal 
cells  instead  of  the  "  limiting 
cells  "  of  other  writers,  are  in 
contact  proximally  with  pro- 


VISUAL    TISSUES  ^53 

cesses  from  connective-tissue  cells  which  lie  far  below  the  basal  mem- 
brane and  the  visual  cells.  These  processes  often  contain  intra-cellular 
blood  capillaries,  and  sometimes  the  connective-tissue  nucleus  is  found 
in  them.  The  line  of  contact  between  an  accessory  cell  and  such  a 
connective-tissue  process  is  always  on  the  basal  membrane. 

The  visual  cells  in  the  adult  have  their  principal  cell  body  and  nucleus 
far  below  the  basement  membrane  (Fig.  223,  A,  B).  The  proximal  end  of 
each  is  produced  into  a  nerve  fiber,  and  the  distal  end,  just  above  the 
accessory  retinal  cells,  gives  rise  to  a  long,  wide,  and  round-ended  visual 
rod.  The  most  important  structure  in  the  eye  is  the  neuro-fibril,  which 
enters  the  cell  body  and  passes  through  the  cell  to  enter  into  the  visual 
rod  as  an  axial  filament.  Here  it  traverses  the  whole  length  of  the  rod 
in  a  somewhat  sinuous  course  and  terminates  as  a  knob  in  the  tip.  The 
relations  of  the  pigmented  cytoplasm  of  the  accessory  retinal  cells  to  this 
axial  filament  and  end-knob  are  most  interesting.  When  in  the  dark, 
the  pigment  is  all  to  be  found  in  the  pigment  band,  which  marks  the  distal 
ends  of  the  accessory  cells.  As  the  light  increases,  the  pigment-bearing 
cytoplasm  of  these  cells  travels  in  a  thin  stream  that  surrounds  the  fibril 
until  it  reaches  the  knob  (Fig.  223,  A,  represents  an  intermediate 
stage).  When  the  light  is  brighter,  as  in  direct  sunlight,  the  knob  itself 
is  surrounded  and  appears  as  a  brown  lump.  This  last  condition  is 
seen  in  a  single  rod  from  Octopus,  pictured  as  Figure  223,  B. 

We  should  speak  here  of  the  remarkable  condition  found  in  the  tetra- 
branch  cephalopods.  The  eye  of  nautilus  is  much  like  the  squid's  as  to 
retina,  but  all  the  complex  accessory  apparatus  is  wanting.  No  lens  or 
cornea  is  present,  and  yet  the  well-developed  retina  has  an  image  pro- 
jected upon  it  by  the  way  its  mouth  of  invagination  is  arranged  as  a 
pin-hole  camera.  Through  this  .hole  the  image  is  formed  as  in  a 
camera  lucida.  The  eye-sac  is  full  of  sea  water  and  in  constant  com- 
munication with  the  water. 

As  Nautilus  is  a  more  rudimentary  form,  it  may  be  asked  if  this 
represents  a  more  primative  condition.  It  may,  as  it  is  analogous  at 
least  to  such  eyes  as  those  of  Patella  and  Trochus.  But  we  must  remem- 
ber that  the  surviving  members  of  ancient  races  frequently  show1  marked 
degenerations  which  amount  to  simplifications,  and  it  is  quite  possible  that 
Nautilus  shows  such  a  condition  in  its  eye.  Its  retina  suggests  the  latter. 

The  mammalian  eye,  which  represents  to  a  degree  all  vertebrate  eyes, 
is  not  only  as  complex  histologically  as  the  cephalopod  eye,  but  it  is  more 
highly  organized  and  is  probably  the  most  efficient  eye  in  existence.  In 
its  derivation  from  the  embryonic  tissues  it  is  unique,  and  we  shall  under- 
stand it  best  by  studying  it  from  that  point  of  view. 

The  most  distinctive  feature  of  its  development  consists  in  the  fact 
that  two  principal  tissues  take  part  in  forming  it  jointly.  The  retinal 


254 


HISTOLOGY 


layer,  instead  of  being  invaginated  from  the  site  of  the  future  eye,  is 
invaginated  from  a  wall  of  the  neural  tube  which  lies  under  the  site  of 
the  future  eyes;  thus  it  is  seen  that  the  retina  is  a  part  of  the  brain's  wall 
(Fig.  224,  A).  It  must  be  remembered  in  this  connection  that  the  brain 
tube  was  itself  an  invagination  of  the  original  ectoderm. 

The  neural  invagination  reaches  toward  the  skin  in  a  cup-like  form, 
and  at  the  same  time  a  thickened  area  of  this  skin  forms  a  depression 


FIG.  224.  —  Five  sketches  to  represent  five  stages  in  the  development  of  the  rabbit's  eye.    (From 
"  STOHR'S  Text-book  of  Histology  "  by  LEWIS.) 

which  advances  inward  to  meet  it  (Fig.  224,  B).  This  latter  structure 
becomes  the  lens  by  being  constricted  off  as  a  sac  from  the  external 
epithelium  (Fig.  224,  C),  and  undergoing  a  thickening  of  its  posterior 
wall  of  epithelium,  until  this  wall  fills  the  sac  up  solid  with  its  long,  paral- 
lel, fiber-like  cells.  Its  nuclei  thus  form  a  row  across  the  middle  of  a 
section  of  the  lens  (Fig.  224,  D,  E),  and  the  anterior  wall  becomes  a 
layer  of  simple  epithelium  covering  the  anterior  surface. 

The  edges  of  the  optic  cup,  as  the  brain  invagination  is  called,  embrace 


VISUAL    TISSUES 


255 


and  hold  the  lens  in  place  (Fig.  224,  D).  The  rear  wall  of  the  optic  vesi- 
cle becomes  a  pigment  layer,  and  the  anterior  or  external  wall  becomes 
the  visual  layer,  which  is  much  complicated  by  the  differentiation  of  com- 
municatory nerve  cells  as  well  as  sustentacular  cells  out  of  its  epithelium. 

The  corneal  layer  become  separated  from  the  lens  by  a  space  contain- 
ing a  fluid,  the  aqueous  fluid  (Fig.  224,  E).  The  space  behind  the  lens 
and  between  it  and  the  retina  is  filled  with  the  vitreous  body.  This 
material  has  its  origin  in  the  connective  tissue  which  occupies  the  space 
when  the  optic  cup  is  first  formed.  It  develops  blood  vessels  and-  a 
bounding  membrane  which  persists,  while  the  blood  vessels  and  part  of 
the  connective  tissue  atrophy,  leaving  the  space  filled  with  a  fluid  and  a 
few  cells.  The  cornea  is  lined  outwardly  by  a  clear,  stratified  epithelium 
of  some  thickness  and  two  principal  layers.  It  rests  on  a  thick  basal 
membrane.  Under  this  is  found  the  thick,  strong  connective  tissue 
which  is  laminated  and  composed  of  fine  fibrils  of  connective  tissue 
with  long,  flat  nuclei.  Inwardly  is  found  a  somewhat  thinner  basal 
membrane  on  which  lies  a  thin  and  single-layered  epithelium  which 
is  in  contact  with  the  fluid  that  fills  the  anterior  chamber  of  the  eye. 

The  lids  are  two 
folds  which  arise  from 
evaginations  and  arc 
analogous  to  the 
squid's  cornea,  while 
the  vertebrate  cornea 
would  have  to  be  com- 
pared superficially,  as 
to  its  origin,  with  the 
squid's  iris.  Glands 
are  found  in  the  lids, 
and  a  large  compound 
tubular  gland,  the 
lachrymal  gland,  is 
placed  near  the  eye 
to  pour  a  lubricating, 
cleansing,  and  moist- 
ening fluid  out  upon 
its  corneal  surface. 

The  retina  is  the 
most  interesting  layer 
and  is  very  complex. 
We  shall  study  it 
by  describing  or,  more  properly,  enumerating  its  various  apparent 
layers  in  order,  and  then  explaining  the  meaning  of  some  of  them  as 


g  g.c.l. 


nv.  f.  I. 


FIG.  225.  —  Section  to  show  the  apparent  layers  of  the  human 
retina,  nv.  f.l.,  nerve  fiber  layer;  g.c.l.,  ganglion  cell  layer; 
in.nJ.,  inner  nuclear  layer;  H.fi.l.,  Henle's  fibrous  layer; 
o.nu.l.,  outer  nuclear  layer;  r.  &  c.,  rods  and  cones;  pg.,  single 
layer  of  pigment  cells;  bl.v.,  blood  vessels.  (From  "STOHR'S 
Text-book  of  Histology"  by  LEWIS.) 


256 


HISTOLOGY 


indicated  by  the  special  processes  of  staining  (Fig.  225).  The  inner- 
most layer,  which  is  a  distal  region  when  explained  embryologically, 
is  a  layer  of  nerve  fibers  passing  in  many  directions 
and  all  terminating  at  the  blind  spot,  where  they 
dip  down  through  all  the  other  layers  to  become 
the  optic  nerve.  Just  under  this  layer  is  found  a 
scattered  layer  of  ganglion  cells,  which  form  the 
ganglion-cell  layer.  This  last  is  followed  by  a  thicker 
fibrous  layer  termed  the  inner  nuclear  layer.  This 
is  composed  of  many  small,  round  nuclei  about  five 
deep.  On  its  outer  margin  comes  the  thin  inner  retic- 
ular  layer,  and  this  is  marked  off  from  the  thicker 
Henle's  fiber  layer  by  an  incomplete  membrane. 

Next  to  Henle's  layer  is  found  the  outer  nuclear 
layer,  which  is  like  the  inner  except  that  it  is  thicker, 
being  about  six  or  seven  nuclei  deep.  Its  outer 
side  is  marked  by  the  sharp  and  clear  outer  lim- 
iting membrane.  From  this  membrane  the  rods 
and  cones  of  the  eye  project  distally  as  a  thicker 
layer.  They  are  both  of  equal  height,  and  each  is 
composed  of  two  segments.  The  rod  segments  are 
about  equal  in  length  and  show  no  great  differen- 
tiation. With  the  cone  it  is  different,  for  the 
basal  segment  is  of  huge  cone-shaped  bulk,  many 
times  as  thick  as  the  rods ;  while  the  distal  segment 
is  very  much  like  that  of  the  rod. 

The  rods  and  cones  are  particularly  well  seen 
in  some  fish,  as  in  the  herring.  Figure  226  repre- 

FIG.  226.  — Part  of  the  sents  a  cone  an(^  several  r°ds  from  this  fish.    The 
retina  of  a  herring,  show-  cone  is  not  as  high  proportionally  as  in  man,  and 

;nrgansel7wkhrctt°hfeCvt  P"?  of  the  ™^  *^™*  has  been  cut  off  in  pre- 
uai  cells  perceive  light,  paring  the  section.  This  preparation  is  particu- 
Ten  rods  (r&.)  and  one  j^  j  happy  in  showing  the  transverse  lamellae  of 

cone    (c.).      nu.,  nuclei  /          \rJ  6 

of  the  rod  cells.   TWO  which    the    rods    are    composed.    The   rods  and 

nuclei  visible  in  the  cone  cones  form  the  distal  layer  Qf  this  waU  Qf  the 
cell,  x  1000.  .... 

ongmal  optic  cup,  or  eye-sac.  The  opposite  wall, 
however,  has  been  closed  in  and  intimately  applied  to  them,  so  that  it 
appears  to  be  and  is  indeed  another  layer  of  the  retina.  It  is  a  simple 
layer  of  cuboidal  cells,  filled  with  pigment  in  which  the  distal  ends  of  the 
rods  are  buried.  When  the  light  becomes  too  bright,  the  cytoplasm  of 
these  cells  is  pushed  up  between  the  rods,  and  thus  it  partially  shades 
them.  This  is  a  very  different  condition  from  that  found  in  many  other 
eyes. 


VISUAL   TISSUES 


257 


vts. 


Some  explanation  of  the  real  relations  existing  between  these  different 
layers  is  highly  necessary.  Figure  227  is  a  diagram  which  explains  in 
part  what  has  already  been  discovered.  As  in  other  retinas  the  rods  and 
cones  are  the  visual 
cell-organs.  The 
visual  cells  them- 
selves form  a  thick 
layer  below  the 
external  limiting 
membrane,  and 
their  nuclei  form 
the  outer  nuclear 
layer.  The  bodies 
of  the  cone  cells 
are  thicker  than 
those  of  the  rod 
cells,  and  their 
bases  are  more 
greatly  expanded. 
The  line  of  these 
bases  is  the  mem- 
brane-like line  be- 


9-c. 


w./. 


sup.  c. 


FIG.  227.  —  Diagram  of  some  of  the  known  elements  of  the  retina 
in  man.  Compare  with  Fig.  225.  vis.c.,  layer  of  visual  cells 
whose  perceptory  organs  form  the  rod  and  cone  layer  and  whose 
nuclei  and  processes  form  the  outer  nuclear  layer  and  Henle's 
layer;  nv.c.,  layer  of  bipolar  and  amakrine  nerve  cells  whose 
nuclei  form  the  inner  nuclear  in  ordinary  preparations;  g.c., 
ganglion  cells  of  the  ganglion  cell  layer;  nv.f.,  nerve  fiber  layer; 
sup.c.,  supporting  or  neuroglia  cells.  (From  "  STOHR'S  Text-book 
of  Histology  "  by  LEWIS.) 


tween  the  inner 
reticular  layer  and 
Henle's  layer  of 
fibers. 

The  other  cellular  layers  represent  a  number  of  different  kinds  of 
nerve  cells  whose  processes  make  various  intricate  connections.  The 
use  of  these  connections  is  obscure  and  not  really  known.  All  that  can 
be  said  is  that  the  various  cells  act  in  some  way  together  and  correlate  the 
various  impulses  in  a  way  that  prepares  them  to  represent  the  image  to 
the  brain.  Some  of  these  nerve-cell  relations  are  shown  in  the  diagram 
represented  by  Figure  227. 

There  are  sustentacular  elements  in  the  retina  of  man  which  extend 
from  the  fiber  layer  to  the  rods  and  cones,  and  serve  to  hold  all  together. 
These  are  known  as  the  radial  fibers,  and  two  of  them  are  pictured  in  the 
diagram.  They  are  neuroglia  elements. 

The  connective  tissue,  muscle,  and  other  histological  details  of  the 
outer  parts  of  the  eyeball  as  well  as  of  the  iris  and  sclera  cannot  be 
taken  up  here.  Many  blood  vessels  penetrate  all  parts  of  the  eye,  in- 
cluding the  retina,  to  supply  its  various  needs. 

Technic.  —  Eye  preparation  presents  the  greatest  possible  variety 
of  technic,  owing  to  the  very  great  variety  in  the  different  kinds  of  eyes. 


258  HISTOLOGY 

The  nerve  elements  demand  the  skillful  use  of  silver  and  methylene  blue. 
The  combined  maceration  and  section  method  outlined  in  the  last  chap- 
ter is  useful  in  studying  the  retina  and  visual  epithelia  of  many  eyes. 
For  the  more  general  histological  relations  the  ordinary  paraffin  sections 
will  do  when  the  eye  is  one  of  the  simple  ones  whose  tissues  are  homo- 
geneous. In  perhaps  the  majority  of  cases,  however,  the  lens  acts  as  a 
formidable  obstacle  to  the  securing  of  perfect  or  even  fair  sections.  The 
lens  can  seldom  be  softened,  and  the  best  way  is  to  remove  it  by  careful 
dissection  after  the  tissue  is  fixed  and  before  the  embedding  has  been 
begun.  With  the  lens  removed  very  good  general  sections  can  be  cut, 
especially  in  celloidin,  in  which  case,  however,  the  sections  are  apt  to  be 
too  thick.  In  the  arthropod  eye  the  difficulty  is  not  the  lens  but  the 
cuticle,  and  this  is  not  so  formidable  an  obstacle.  The  cuticle  can  some- 
times be  removed,  especially  when  it  is  very  thick.  At  other  times  it 
can  be  cut  in  the  case  of  a  thin-shelled  or  newly  moulted  animal.  For 
general  studies  of  the  tissues  Zenker's  fluid  or  chrom-aceto-formol  is 
the  best.  For  a  good  picture  of  the  retina  Flemming's  strong  fixative 
or  some  other  fluid  containing  osmic  acid  is  the  best. 

LITERATURE 

ENTZ,  G.     "  tiber  Inf  usiorien  des  Golfe  von  Neapel,"  Mil.  d.  Zool.  St.  zu  Neapel,  Band  V, 

1884. 

SHARP,  B.     "The  Eyes  of  Lamellabranchiata,"  Mi.  zu  Neapel,  1886. 
SCHEWIAKOFF,  WALD.     "  Beitrage  zur  Kentniss  des  Acalephanges,"  Morph.  Jahrb.,  Band 

XV,  pp.  21-60,  1889. 

PFEFFER,  W.     "Die  Sehorgane  der  Seesterne,"  Zool.  Jahrb.,  Band  XIV,  1901. 
HESSE,  RICH.     Articles  on  the  "Eyes  of  Invertebrates"  in  Zeits.  f.  Wiss.  Zool.,  several 

recent  volumes. 

WATASE,  S.     "The  Eye  of  Limulus,"  Studies  from  the  Biol.  Lab.,  Johns  Hopkins  Uni- 
versity, Vol.  IV,  No.  6. 
SARASIN,  P.  B.  and  C.  F.     "  Uber  einen  mit  Zusammengesetzten  Augen  gedeckten  Seeigel," 

Zool.  Am.,  Band  VIII,  Nr.  an,  1885. 
PHILLIPS,  E.  F.     "Structure  and  Development  of  the  Compound  Eye  of  the  Bee,"  Proc. 

of  the  Acad.  N.  Sc.,  Philadelphia,  February,  1905. 

CAJAL,  RAMON  Y.  .."Le  Retine  des  Vertebres,"  La  Cellule,  Vol.  IX,  1893. 
DOGIEL,  A.  S.     "Uber  die  nervosen  Elemente  in  der  Retina  des   Menchen,"  Arch.  f. 

mik.  Anat.,  Band  XXXVIII. 

BERNARD.     "  Studies  in  the  Retina,"  Quart.  Journ.  Micr.  Sc.,  1902  and  1903. 
REDIKOOZEW,  W.     "  Untersuchungen  uber  den  Bau  der  Ocellen  der  Insekten,"  Zeits.  f. 

Wiss.  Zool.,  Band  LXVIII,  1900. 


THE   OLFACTORY  AND   GUSTATORY   NERVE   TISSUES 

The  olfactory  and  gustatory  nerve  cells  form  a  comparatively  small 
group  of  perceptory  neurons,  which  are  distinguished  from  the  others  by 
the  fact  that  they  can  receive  impressions  directly  from  the  atoms  and 
molecules  of  some  substances  that  have  been  dissolved  in  the  atmosphere 


GUSTATORY  AND   OLFACTORY  TISSUES  259 

or  in  water.  These  impressions  are  such  as  distinguish  the  quality  or 
individuality  of  the  substance,  and  only  a  certain  proportion  of  known 
substances  can  produce  an  impression,  at  least  in  the  case  of  man.  Many 
different  substances  produce  similar  impressions,  and  would  be  identified 
as  the  same  by  most  animals. 

The  power  is  one  that  we  do  not  fully  understand,  partly  because  our 
own  organs  of  taste  and  smell  are  so  poorly  developed.  The  delicacy  of 
this  perception  in  other  animals  is  wonderful  beyond  belief,  while  in 
still  others  it  is  very  poorly  developed  or  not  at  all.  In  the  case  of  man 
it  is  probably  degenerate  from  a  former  condition  of  high  efficiency. 

The  cells  that  perform  the  function  are  of  necessity  epithelial  in 
character,  and  in  the  cases  where  we  have  seen  them  they  are  usually 
very  thin  and  elongate.  There  is  no  peculiar  structure  of  their  sensory 
ends  by  which  they  can  always  be  distinguished  from  some  other  per- 
ceptory  cells  of  the  simpler  types  that  we  know  certainly  to  be  tactile  or 
other  cells.  In  both  we  meet  with  a  variety  of  rod-  or  hair-like  perceptory 
organs.  In  all  cases  the  function  must  be  determined  either  by  expe- 
rience, which  we  can  only  do  in  man;  or  by  homo  logy,  as  is  done  in  the 
other  classes  of  vertebrates ;  or  by  experiment  and  observation,  as  must 
be  done  in  all  other  animals.  Thus  we  cannot  be  sure  of  the  exact 
function  of  many  of  the  organs  that  have  been  given  the  name  of  "  ol- 
factory "  or  "  gustatory  "  organs  in  a  large  number  of  lower  forms. 

In  the  vertebrates  we  find  the  gustatory  and  olfactory  cells  forming 
two  distinct  types,  the  only  strong  bond  between  them  being  the  fact  that 
they  are  both  used  to  perceive  chemical  qualities. 

The  olfactory  cells  are  the  more  typical  of  the  two  and  so  similar  in 
all  forms  to  those  of  the  birds  that  we  shall  study  them  as  found  in  the 
common  fowl,  comparing  them  with  the  well-known  form  of  man.  The 
epithelium  on  the  olfactory  prominence  of  the  chicken  (Fig.  228)  consists 
of  the  same  three  sorts  of  cells  found  in  the  mammals,  a  sustentacular 
group,  the  more  numerous  in  number,  and  a  liberal  number  of  olfactory 
cells  lying  scattered  among  them,  the  whole  resting  on  a  layer  of  basal 
cells.  A  basement  membrane  is  so  weakly  developed  as  to  be  apparently 
absent. 

The  sustentacular  or  supporting  cells  have  long  bodies  reaching  from 
the  surface  down  to  the  peculiar  layer  of  basal  cells.  Here  the  proximal 
end  of  the  cell  branches  once  or  twice,  and  its  ends  are  attached  to  and 
intermingled  with  the  processes  of  the  basal  cells.  The  cell  body  is  of 
an  irregular  form,  expanded  to  contain  the  nucleus  somewhere  in  its 
distal  two  thirds,  and  it  does  not  show  the  strong  granular  differentiation 
of  proximal  and  distal  cytoplasm  that  these  cells  do  in  the  human  tissues, 
except  that  the  distal  end  secretes  mucus  in  small  quantities,  and  in  some 
cases  is  apparently  ciliated.  False  appearances  of  ciliation  are  to  be 


260 


HISTOLOGY 


seen  in  parts  of  the  mucus  that  covers  the  epithelium,  as  is  also  the  case 
in  the  mammals.  The  distal  ends  of  the  sustentacular  cells  form  an 
even  surface.  The  pigment  that  gives  the  characteristic  yellow  or  brown 
color  to  this  surface  in  so  many  forms  of  vertebrates  is  here  deposited  as 
a  thin  layer  in  the  outermost  parts  of  these  cells.  The  nuclei  are  easily 
distinguished  from  the  nuclei  of  olfactory  cells  by  an  oval  form  and 
smaller  size  as  well  as  a  different  chromatin  pattern.  They  lie  in  a  broad 
zone  of  the  epithelium  composed  of  the  distal  two  thirds .  of  the  cells, 
and  are  therefore  to  be  found  at  varying  heights  in  the  cell.  In  the 


FIG.  228.  —  Bit  of  olfactory  epithelium  from  the  fowl,  Callus  domesticus .  Seen  in  situ  to  left. 
Several  individual  cells  shown  separately  to  right,  sup.c.,  supporting  cells;  sen.c.,  sen- 
sory cells;  ba.c.,  basal  cells. 


mammals  they  are  almost  always  found  at  one  height  in  the  cell,  and 
therefore  lie  in  a  single  plane  and  appear  as  a  single  row  in  section. 

The  third  kind  of  cells  in  this  tissue,  the  basal  cells,  are  ectodermal 
in  origin,  and  therefore  an  integral  part  of  the  epithelium.  In  the  fowl 
this  fact  does  not  appear  to  advantage,  the  cells  looking  as  much  like 
branching  connective-tissue  cells  as  anything  else.  This  appearance  is 
intensified  by  the  lack  of  a  clearly  defined  basement  membrane.  The 
cytoplasm  of  the  basal  cells  is  granular  and  branching  and  lacks  definite 
boundaries.  It  forms  a  reticulum  with  the  cytoplasm  of  its  neighbors, 
and  the  other  epithelial  elements  rest  upon  it.  Iri  man  the  basal  cells  lie 
in  a  very  much  narrower  row  between  the  bases  of  the  supporting  and 
olfactory  cells. 

The  olfactory  cells  are  to  be  seen  lying  between  the  sustentacular 


GUSTATORY  AND    OLFACTORY   TISSUES 


26l 


cells,  with  their  round,  clearer,  nucleated  bodies  below  the  level  of  the 
lowest  nuclei  of  the  sustentacular  cells.  The  well-rounded  nuclei  almost 
fill  this  cell  body  (Fig.  228,  sen.  c).  Some  few  have  the  body  up  in  the 
outer  third  of  the  epithelium.  They  give  the  lower  third  of  the  epithe- 
lium a  distinctly  lighter  appearance. 

The  cell  body  is  drawn  out  distally  into  a  thin,  smooth  process  that 
reaches  to  the  surface  and  acts  as  the  perceptory  organ.  Perceptory 
rods  or  hairs  are  not  to  be  distinguished.  The  perceptory  process  is  thin- 
ner than  the  distal  processes  of  the  sustentacular  cells,  but  not  so  thin  as 
the  proximal  portion  of  the  cell  to  which  it  belongs,  which  is  drawn 
out  into  a  much  thinner  and  longer 
process,  the  centripetal  nerve  fiber. 
This  fiber,  which  is  never  myeli- 
nated,  passes  entirely  out  of  the 
epithelium,  and  running  through 
connective  tissue  and  bone,  it  ends 
in  the  central  nervous  system,  as 
in  man,  in  the  olfactory  bulb. 
Here  the  fibers,  in  mammals,  end 
in  peculiar  round  bodies  known  as 
the  glomeruli.  These  are  not  cel- 
lular structures,  but  are  formed  by 


nv.f. 


the    branching    ends    of    the    fibers    FIG.  229.- Silver  picture  of  the  olfactory  cells 
from  the  Olfactory  Cells  United  with        in    the    nasal    epithelium    of    Tropidonotus. 

similar  branching  end-organs  from     ^"EST^^S"*'  "*   ""* 

nerve  cells  in  the  bulb,  the  mitral 

cells,  and  others.     Figure  229  shows  these  relations  by  means  of  the 

silver  process  used  on  an  embryonic  Tropidonotus. 

The  relation  of  the  olfactory  cell  to  the  second  kind  of  chemico- 
perceptory  cell  that  we  shall  study,  the  gustatory  cell,  or  cell  of  taste, 
is  a  close  one  as  regards  function.  In  regard  to  its  origin  and  structure, 
however,  it  may  be  strongly  contrasted  with  it.  Both  perform  the  same 
function,  but  do  so  in  different  ways  and  attain  different  results.  The 
olfactory  cells  deal  with  the  finely  divided  atoms  or  molecules  of  sub- 
stances in  a  gaseous  state  and  usually  dissolved  in  air  or  water.  The 
gustatory  cells,  at  least  those  of  man,  require  more  crude  masses  of  the 
substance  and  require  an  immediate  contact  with  them.  Also  they 
most  probably  must  always  be  dissolved  in  water  or  some  fluid  of  which 
water  is  a  part. 

The  result  is  less  definitive  and  delicate  from  the  stimulation  of  a 
gustatory  cell.  We  cannot,  in  this  way,  detect  fine  flavors  and  aromas, 
as  can  be  done  from  olfactory  cells,  but  secure  only  a  few  coarse  impres- 
sions as  sweet,  sour,  bitter,  the  warmth  impression  of  alcohol,  and  such 


262 


HISTOLOGY 


tastes.    The  "  taste  "  of  our  food  is  largely  composed  of  olfactory  im- 
pressions from  the  nose. 

In  structure  the  specific  cells  of  the  gustatory  tissues  are  very  different, 
in  the  vertebrates,  from  those  of  the  olfactory  organs.  Their  discharging 
end  is  not  the  extremity  of  a  process  extending  into  the  brain.  Instead, 
the  centripetal  end  of  the  cell  body  is  rounded,  and  the  receiving  nerve 
cells  in  the  nerve  center  send  afferent  nerve  fibers  to  the  gustatory  cells 
to  receive  the  stimulus  from  the  cell  body. 

The  gustatory  cells  are  not  found  scattered  among  an  epithelium  as 
are  the  olfactory  cells,  but  associated  in  small  numbers  with  certain 
supporting  cells  that  somewhat  resemble  them  in  shape.  Both  of  these, 
in  the  vertebrates,  may  be  looked  upon  as  simple  columnar  epithelial 
elements  that  were  prevented  by  their  function  from  developing,  along 
with  the  rest  of  a  former  simple  epithelium,  into  the  stratified  form. 
These  small  collections  of  taste  cells,  with  their  supporting  cells,  form 
structures  known  as  "taste  buds  "  and  are  to  be  found  in  various  parts  of 
the  mouth  in  the  higher  vertebrates ;  while  in  the  Amphibians  and  fishes 
they  enjoy  a  wider  distribution,  even  out  to  the  face  and  head  or  on 
fin-rays  and  barbies  developed  for  this  purpose.  In  man  they  are  to  be 
found  on  the  vallate  papillae,  the  sides  of  the  tongue,  and  on  the  outer 
surface  of  the  epiglottis. 

The  specific  cell  of  the  tissue  is  a  thin  and  elongated  epithelial  form 
reaching  from  the  surface  to  the  basement  membrane  where,  unlike 

the  olfactory  cell,  it  ends.  The 
cell  is  somewhat  enlarged  at  its 
middle  or  lower  part  to  contain 
the  nucleus.  Its  distal  end  bears 
a  short  rod-like  structure,  the  cell- 
organ  of  perception  (Fig.  230). 
There  is  but  little  variation  in  the 
shape  of  this  kind  of  cell  through- 
out the  vertebrate  groups.  One 
can  see  it  thinner  and  longer  in 
the  skin  of  Lamperta,  or  shorter 
and  thicker  in  the  skin  of  several 
fishes.  Its  arrangements  as  a  tis- 
sue with  the  accessory  supporting 
FIG.  23o. -Taste  bud  in  tongue  of  man.  g.c.,  cells  is  more  interesting.  The  two 

gustatory  cells;  sup.c.,  supporting  cells;  nv.fi.,     together  form  isolated  groups,  with 

the  specific  cells  tending  toconcen- 
trate  at  the  center  of  each  group. 
A  rather  diffuse  group  of  this  kind  is  formed  in  the  larval  form  of 
Petromyzon.  In  this  case  the  thread-like  taste  cells  are  scattered 


g.c. 


sup 


GUSTATORY  AND    OLFACTORY   TISSUES  263 

among  the  heavy  and  far  thicker  supporting  cells,  which  also  have 
much  larger  nuclei.  In  most  vertebrates  the  gustatory  cells  are  few 
in  number,  and  are  collected  into  the  central  portion  of  the  "bud," 
while  the  supporting  cells  surround  it.  In  this  position  the  wider  or 
thicker  middle  portions  of  these  cells,  together  with  the  narrow  proximal 
ends  and  the  still  more  narrow  distal  ends,  make  the  whole  organ  oval  in 
outline  or  melon-shaped. 

The  ends  of  the  supporting  cells  are  not  arranged  evenly  with  the 
surface  of  the  surrounding  epithelium,  but  form  a  depression  or  pit  into 
which  the  gustatory  rods  of  the  taste  cells  project.  The  nerve  supply 
comes  as  a  series  of  afferent  fibers  from  cells  in  or  connected  with  the 
brain  centers.  These  fibers  enter  the  taste  bud  and  end  in  lateral  con- 
tact with  the  gustatory  cells.  The  impressions  received  by  the  cells  in 
their  contact  with  the  food  or  other  substances  are  transmitted  as  im- 
pulses to  these  nerve  ends  which  carry  it  to  the  nerve  centers.  In  the 
mammalian  taste  organs  some  of  the  nerve  endings  enter  the  epithelium 
near  the  taste  bud,  but  not  in  contact  with  the  sense  cells.  While  the  real 
sensory  endings  are  thicker  and  of  irregular  outline,  these  outer  ones  are 
thin  and  of  smooth  contour.  They  probably  do  not  convey  any  gustatory 
sensation. 

Another  group  of  animals  in  which  we  can  be  almost  certain  the 
sense  of  smell  and  taste  are  present  are  the  insects.  These  creatures 
have  been  proven  by  experiment  to  be  able  to  smell  much  more  keenly 
than  man,  and  perhaps  are  better  able  to  smell  delicate  and  diffuse  odors 
than  the  keen-scented  mammals.  A  dog  knows  many  individual  persons 
by  their  smell,  but  an  ant  is  able  to  distinguish  all  the  members  of  its 
colony  instantly  by  their  smell  and  to  make  the  power  serve  him  almost 
as  usefully  as  sight  serves  man.  The  carrion  insects,  beetles,  blowflies, 
etc.,  can  detect  the  presence  of  and  find  their  way  to  any  decaying  pro- 
toplasm as  certainly  and  quickly  as  many  other  mammals  find  their  way 
to  food  by  sight  or  a  knowledge  of  its  location.  Among  many  forms 
of  insects  (as  also  some  mammals)  the  females  in  the  breeding  season 
emit  an  odor  that  attracts  the  males  from  many  miles  away.  This  is 
particularly  true  of  some  night  Lepidoptera.  The  sense  of  taste  has  also 
been  proven  by  experiment  to  be  present  in  many  insect  forms. 

We  shall  first  study  the  olfactory  tissues  of  insects.  These  organs 
always  are  found  on  the  antenna  among  certain  possible  tactile  and  other 
sense  organs.  The  real  olfactory  organ  is  a  hypodermis  cell  which  has 
been  differentiated  into  a  nerve  cell  capable  of  being  stimulated  by 
certain  odors  (substances  in  a  state  of  fine  division  in  the  air  and,  possibly, 
the  water).  This  perceptory  nerve  cell  has  its  proximal  end  (efferent 
end)  drawn  out  into  a  nerve  fiber  which  passes  into  the  body  to  enter 
some  ganglion. 


264 


HISTOLOGY 


The  distal  end  of  the  cell  is  specialized  into  a  distinct  rod  or  cell- 
organ  of  smell.    This  rod  may  be  clothed  by  a  cuticular  and  hair-shaped 

cap,  or  it  may  be  found  at  the  bot- 
tom of  a  depression  or  "  pit "  whose 
object  is  to  protect  it.  This  pit  is 
formed  by  the  surrounding  hypoder- 
mis,  and  is  sometimes  almost  covered 
over  on  the  top.  The  olfactory  cell 
is  sometimes  very  large  and  multi- 
nuclear. 

The  olfactory  cells  of  the  carrion 
beetle,  Necrophorus.  This  insect  is 
one  of  the  first  to  arrive  on  the  scene 
when  an  animal  dies  and  begins 
to  decay.  The  beetle  has  antennas 
whose  terminal  joints  are  provided 
with  parallel  plates.  The  surfaces  of 
these  plates  are  lined  with  hypoder- 
mal  cells  that  have  built  the  greater 
part  of  the  heavy  chitinous  wall. 
Among  them  lie  the  sense  cells  of 
smell,  and  some,  perhaps,  of  touch 
also.  Together,  in  our  transverse 
section  (Fig.  231),  they  form  a  very 
thick,  apparently  single,  layer  of  epi- 
thelium, whose  cells  are  so  long  and 
narrow  that  their  exact  boundaries 
cannot  be  easily  determined.  Occu- 
pying the  center  of  the  plates  is  a 

FIG.  231.  — Olfactory  epithelium  from  the      w      &     .  . 

antenna  of  a  carrion  beetle,  Necrophorus.      niaSS  of  COnnCCtlVC-tlSSUC  Cells,  nerve 

cu.,  outer  layer  of  the  elaborate  cuticle;    fibers,  and  some  large  and  very  re- 

i.cu.,  inner  layer  of  the  same;  hy.c.,  hypo-  i     11          11         i 

dermal  cells;  sen.c.,  sensory  cells;   ol.or.,      markable  Cells  whose   exact    meaning 


hy.  c. 


nv.f. 


olfactory  perceptory  organs  of  the  sensory 
cells;  nv.f.,  nerve  fibers  coming  from  sen- 
sory cells;  t.h.,  tactile  hair;  gl.c.,  gland 
cells,  whose  ducts  pass  through  the  olfac- 
tory epithelium  to  open  through  the  cu- 
ticle. X  950. 


is  not  clear. 

The  epithelium  should  first  be 
studied.  It  shows  plainly  two  kinds 
of  cells.  The  thinnest  is  the  wall-form- 
ing hypodermis  cell,  whose  smaller 
and  narrower  oval  nucleus  shows  a  compact  granular  chromatin  pattern. 
The  cuticle  is  peculiar.  It  may  be  said  to  have  two  layers.  The 
outer  layer  is  chitinous,  but  at  many  points  it  is  depressed  into  deep  pits. 
These  pits  do  not  open  altogether  through  to  the  inside,  but  have  their 
lower  floor  everted  into  the  cavity  so  that  it  lines  the  pit  as  a  second  and 
very  thin  wall  whose  entire  surface  is  perfect.  Other,  and  less  deep, 


GUSTATORY  AND    OLFACTORY   TISSUES  26$ 

pits  have  this  inner  wall  arising  from  a  higher  or  more  distal  height  and 
everted  into  a  long,  sharp,  curved  hair  instead  of  the  round,  club-shaped 
organ  seen  in  the  deeper  pit.  The  proximal  layer  is  of  a  different  kind 
of  chitin  and  stains  a  different  shade  altogether,  with  most  stains.  It  is 
not  a  layer  in  the  proper  sense  of  the  word,  being  a  set  of  proximal  pro- 
jections from  the  real  chitin  layer. 

The  nerve  cells  are  readily  recognized  among  the  hypodermal  cells 
by  their  thicker  body  and  larger,  clearer,  and  rounder  nucleus.  They  lie 
with  their  main  cell  body  in  the  proximal  third  of  the  layer,  and  thus  their 
nuclei  are  found  among  those  of  the  hypodermis  cells.  The  cell  body  is 
drawn  out  distally  and  enters  into  one  of  the  pit  knobs  or  hairs.  Both 
kinds  are  sensory,  and  knowing  what  we  do  about  the  tactile  hairs  of 
insects,  it  is  fair  to  believe  that  the  hair-covered  endings  are  tactile,  while 
the  knob-covered  endings  are  olfactory. 

The  knob-covered  endings  which  rest  in  the  deep  pits  are  entirely 
homologous  with  the  hairs.  They  are  but  shorter,  blunter,  and  more 
deeply  set  hairs.  They  also  exactly  represent  the  "  pegs  "  and  other 
olfactory  processes  of  Hauser,  Graber,  and  many  other  writers.  The 
nerve-cell  cytoplasm  which  they  contain  is  granular  and  represents  a 
special  cell  organ  of  olfactory  perception.  It  is  aways  protected  from 
the  exterior  by  the  cuticle,  and  in  no  case  were  the  writers  able  to  find  a 
case  where  the  cytoplasm  of  either  an  olfactory  pit,  peg,  knob  or  hair 
organ  had  direct  access  to  the  air.  At  first  the  broken  and  cut  hair 
endings  give  this  impression,  but  it  soon  becomes  easy  to  detect  the  arti- 
fact. 

The  proximal  end  of  the  cell  is  drawn  out  into  a  nerve  fiber,  which 
runs  in  to  connect  with  some  ganglionic  center.  In  connection  with 
this  nerve  fiber  one  often  sees  a  peculiar  black  line  (in  iron  haematoxylin- 
stained  specimens)  which  enters  the  epithelium  and  passes  directly 
through  it  to  the  cuticle.  Following  this  line  back  out  of  the  epithe- 
lium, it  is  seen  to  reach  one  of  the  large  cells  mentioned  above  and  enter 
into  a  vacuole-like  area  of  this  cell,  where  it  ends  in  a  cylindrical  enlarge- 
ment of  some  little  length.  These  cells  are  large,  with  nuclei  that  are 
round  and  full  and  a  cytoplasm  that  is  granular,  as  in  a  nerve  cell.  At 
first  sight  they  are  liable  to  be  taken  for  nerve  cells,  but  must  probably 
be  considered  as  gland  cells,  and  the  line  for  a  cuticular  tube  which  ends 
in  their  vacuolar  area  much  as  the  homologous  tube  does  in  the  secretory 
cell  of  the  odoriferous  glands  of  Belostoma.  The  end  cylinder  is  sur- 
rounded by  a  thicker  covering  of  some  less  dense  material.  The  per- 
ceptory  or  distal  opening  of  the  line  tubule  could  not  be  made  out. 
These  very  peculiar  organs  have  been  thought  to  be  associated  with 
the  sense  of  smell,  and  the  auditory  and  the  static  functions.  They 
are  probably,  as  far  as  we  can  determine  for  the  present,  cells  that  secrete 


266 


HISTOLOGY 


some  fluid,  of  unknown  function,  which  is  discharged  on  the  surface  of 
the  antenna  plates.  A  variety  of  olfactory  tissue  in  which  the  perceptory 
unit  consists  of  an  enlarged  and  multinucleated  nerve  cell  has  been 
described  in  several  grasshoppers. 

The  organs  of  taste  in  insects  have  been  very  satisfactorily  located 
experimentally,  but  they  have  not  been  differentiated  structurally  from 
the  organ  of  smell.  The  palpi  are  used  to  taste  with,  especially  on 
such  insects  as  use  them  to  touch  food  after  it  is  in  the  mouth. 

The  mollusks  also  have  sensory  organs  which  can  better,  perhaps, 
be  considered  as  olfactory  rather  than  gustatory  structures.  They  may 


FlG.  232. —  Central  part,  and  epithelium  on  one  side,  of  an  osphradium  plate  of  Sycotypus 
canaliculatus ,  transverse  section,  b.m.,  basement  membranes  on  each  side;  sen.ep.,  sen- 
sory epithelium;  res.ep.,  part  of  respiratory  epithelium;  lu.ep.,  portion  of  lubricating 
epithelium  with  one  mucous  cell.  X  700. 

not  always  be  used  to  test  or  locate  food  matter,  but  are  possibly  of  use 
in  testing  the  purity  of  the  water  or  other  of  its  qualities  which  are  of 
importance  to  the  animal. 

In  their  form,  these  cells  are  remarkably  like  the  earlier  stages  of 
the  olfactory  cells  in  the  vertebrate  embryo,  having  their  body  drawn 
out  into  a  short  centrifugal  process  on  which  the  sensory  cell  organ  is 
developed ;  also  into  a  long  centripetal  process,  which  acts  as  the  nerve 
fiber  to  carry  the  impulse  to  the  central  ganglion.  In  regard  to  their 
tissue  organization,  we  may  find  what  appear  to  be  two  kinds  of  organs  : 
one  which  is  found  as  part  of  a  modified  gill,  the  osphradium,  and  an- 
other which  is  developed  from  the  inner,  mantle  epithelium,  in  several 
places,  in  the  cephalopod  mollusks. 


GUSTATORY  AND    OLFACTORY   TISSUES  267 

The  osphradium  of  Sycotypus  canaliculatus  is  a  modified  gill  whose 
parallel  plates  show  much  likeness  to  the  gill-plates  represented 
in  Chapter  XVII.  Figure  232  shows  one  side  of  a  plate  in  vertical 
transection  near  its  base.  To  the  right  is  seen  the  beginning  of  the 
large  extent  of  distal  epithelium,  which  clothes  the  greater  part  of  the 
plate.  Its  tall  columnar  cells  hold  heavy  granules  of  pigment  in  their 
distal  cytoplasm.  The  nuclei  are  at  various  proximal  levels,  and  there 
seems  to  be  a  double  layer  of  irregular  basal  cells  with  rounder  nuclei. 
This  epithelium  is  so  like  the  gill  epithelium  that  one  must  consider  it 
as  respiratory. 

To  the  left  and  nearer  the  body  of  the  animal  is  the  layer  of  still  less 
differentiated  epithelium,  which  lines  the  deep  curvature  of  the  fold. 
It  possesses  some  mucous  cells,  one  of  which  is  pictured,  and  its  nuclei 
are  comparatively  higher  or  more  distal  in  the  cells  than  those  of  the 
outer  epithelium. 

Between  the  two,  and  placed  in  a  longitudinal  belt,  comes  the  olfac- 
tory epithelium.  The  cells  are  longer  and  stouter,  and  their  cytoplasm 
is  clearer.  The  cilia  represent  the  olfactory  rods  seen  in  many  other 
gastropods,  as  Segaretus,  where  they  are  short  and  evidently  not  ciliated. 
Their  length  in  Sycotypus  seems  to  be  rather  an  indication  of  low  special- 
ization. The  body  of  the  lamella  and  the  epithelium  on  but  one  side 
of  it  are  represented  in  Figure  232.  The  epithelium  on  the  other  side 
is  symmetrical. 

The  peculiar  organ,  discovered  by  Van  de  Hooven  in  Nautilus  and 
named  after  him,  is  considered  to  be  an  olfactory  organ,  or  one  for  testing 
the  water,  as  mentioned  above.  It  consists  of  a  region  derived  from 
the  mantle  cavity  and  lined  with  a  thick,  heavy  epithelium.  Where 
a  section  is  teased  and 
mounted,  the  same  effect 
may  be  attained,  as  is  shown 
in  Figure  233.  The  large, 
heavy  cells  with  proximal 
nuclei  are  gland  cells  and 
secrete  a  mucus. 

Another  set  of  cells,  more 
numerous  and  much  finer,  lie 
between  the  gland  cells  and 

extend      thp      samp     Hktanrp        FlG'  233-  —  Teased  preparation  from  the  olfactory 

me      Same      distance,  epithelium  of  a  Nautilus,     gl.c.,  large  gland  cells 

from     base      tO      distal      edge.  which  have  been  shaken  out  from  among  the  thread- 

These   are   the  nerve  or  sen-        g«^ctory  celb,rfur. ;•»./.,  nerw  fiber.   (After 
sory  cells,  and  they  send  ef- 
ferent processes  toward  the  central  ganglia,  while  their  afferent  pro- 
cesses bear  perceptory  cell  organs.    The  efferent  processes  form  a  heavy 


268  HISTOLOGY 

layer  of  fibers  in  the  connective  tissue  which  lies  just  beneath  the  base- 
ment membrane.  These  perceptory  cells  are  peculiar  nerve  elements 
because  of  the  small  size  of  their  cytoplasmic  bodies,  which  are  barely 
large  enough  to  hold  the  very  small  oval  nucleus. 

Technic.  — Nitrate  of  silver  by  the  rapid  process  and  one  good  method 
of  paraffin  sectioning  is  all  that  is  needed  to  study  the  olfactory  organs, 
although  the  macerated  section  method  is  very  applicable  in  this  epithe- 
lium, also.  Where  the  epithelium  is  covered  with  a  cuticle,  as  in  the 
insects,  the  cuticle  is  almost  always  very  thin  and  forms  no  noticeable 
obstacle  to  the  cutting. 

LITERATURE 

GRABERG,    J.     "Zur   Kenntniss   des   cellulosen   Baues   der   Geschmacksknospen   beim 

Menchen,"  Anat.  Hefte,  Band  XII,  1899,  S.  337. 

NAGEL,  WILIBALD.     "Die  niederen  Sinne  der  Insecten,"  Tubingen,  1892. 
VOMRATH,  O.     Articles  on  the  Nerve-Ending  in  several  recent  volumes  of  Zeits.  f.  Wiss. 

Zool. 


CHAPTER    XIV 
PIGMENT  TISSUES 

ANIMALS,  in  certain  of  their  tissues  and  products  of  these  tissues,  are 
variously  colored.  These  colors  depend  upon  refractive  surfaces,  dif- 
fracting lines  or  markings,  and  inclosed  colored  substances  called  pig- 
ments. The  various  substances  giving  rise  to  the  colors  of  the  tissues 
and  of  their  products  are  included  under  the  term  pigments.  These 
pigments  may  be  inert,  serving  merely  as  coloring  and  protective  devices ; 
or  may  be  of  great  physiological  importance.  They  occur  either  in  a 
diffused  or  in  a  segregated  form.  Considered  as  coloring  substances, 
they  serve  to  protect  from  light  or  to  conceal  from  the  observation  of 
enemies  and  prey  and  to  attract  or  to  warn  other  organisms. 

Examples  of  diffused  pigments  among  animal  products  are  the 
bilirubin  of  the  bile,  the  urochrome  of  the  mammalian  urine,  the  purple 
secretion  of  several  nudibranch  mollusks,  and  the  brown  or  black  secre- 
tion of  the  cephalopod  mollusks.  Other  varieties,  based  upon  their 
physiological  use,  might  be  demanded  by  physiologists  in  a  complete 
classification,  but  the  above  must  suffice  as  examples  of  what  we  mean 
by  diffused  pigments  in  tissue  products.  In  general  we  shall  consider 
here  only  such  pigments  as  apparently  exist  to  serve  the  animal  by  their 
color. 

Many  organisms  have  colors  peculiar  to  them,  which  are  due  to  dif- 
fused pigments  in  the  tissues  themselves.  Among  the  Protozoa  the 
members  of  the  genus  Vampyrella  often  present  a  diffused  red  pigment, 
which  gives  the  specimens  a  characteristic  color.  Many  of  the  colors 
of  the  insects  are  due  to  diffused  pigments.  Myochrome  is  a  diffused 
pigment  that  gives  the  red  color  to  mammalian  muscle.  Perhaps  the 
most  important  diffused  pigment  is  the  haemoglobin  in  the  blood  of 
all  vertebrates  and  some  invertebrates;  in  this  case  the  pigment  is  a 
substance,  the  physiology  of  which  we  understand  and  in  which  the 
color  is,  possibly,  more  of  an  incident  than  a  point  of  any  importance 
to  the  economy  of  the  organism.  It  can  be  said,  however,  that  the  color 
of  blood  does  serve  a  distinct  end.  Other  such  diffused  pigments  of 
various  colors  are  found  in  the  blood  of  invertebrates.  In  many  of 

269 


2/0  HISTOLOGY 

these  cases  the  color  does  not  appear  until  the  blood  is  exposed  to  the 
air  or  to  the  action  of  other  external  conditions. 

A  more  conspicuous  form  of  pigment  from  an  histological  stand- 
point is  that  due  to  the  segregation  of  the  pigment  material  in  the  tissues 
or  their  products.  In  this  condition  the  coloring  matter  is  assembled 
in  little  granules  within  the  cell,  usually  in  the  cytoplasm  which  it  some- 
times fills  completely,  and  at  other  times  it  gathers  in  certain  localities 
called  the  pigmented  areas.  These  granules  appear  to  play  the  same 
rdle  for  the  animal  pigments  that  the  chloroplasts  play  for  the  chlorophyll 
in  most  plants. 

The  so-called  stigmata  of  certain  Protozoa  are  rather  conspicuous 
bodies,  which  bear  a  bright  red  segregated  pigment.  In  other  Protozoa 
there  are  to  be  found  granules  that  bear  a  brown  pigment  that  is  quite  like 
that  of  the  brown  Algae,  as,  for  example,  Crysamceba;  other  granules 
may  be  found  that  contain  a  dark  to  black  pigment  resembling  greatly 
the  melanin  of  higher  forms  (Metopus}.  In  the  invertebrates  these 
dark  pigment  granules  are  very  common.  It  remains  to  be  seen  if  this 
resemblance  to  the  melanin  is  superficial  or  a  real  one.  Melanin  is  the 
most  common  form  of  the  segregated  pigments  found  among  the  higher 
vertebrates.  It  gives  the  characteristic  color  to  the  choroid  of  the  eye 
and  to  the  skin  in  the  darker  members  of  the  human  race.  Other  cells 
inside  the  body  show  it  occasionally,  as  the  nerve  cells  in  age  (see  Fig. 
163)  and  other  cells  in  disease.  In  the  nerve  cells  the  granules  may  not 
be  a  true  melanin,  but  if  it  is,  its  presence  could  be  accounted  for  by  the 
ectodermal  origin  of  these  cells  in  which  the  pigment  was  a  vestigial 
character. 

In  the  tissues  of  the  lungs  segregated  pigments  are  found,  which  are 
important  retainers  of  oxygen.  Segregated  pigments  are  character- 
istic of  the  tissues  of  the  alimentary  tract  and  of  the  liver.  These  are 
quite  active  as  retainers  of  digestion  products. 

Diffused  pigments,  like  urochrome,  may  be  considered  as  excretions 
that  have  been  taken  up  from  the  blood  along  with  the  other  constitu- 
ents of  the  urine.  Bilirubin,  on  the  other  hand,  is  to  be  taken  as  a 
secretion  of  the  liver  cells.  The  pigment  of  the  ink  of  the  cephalopods 
is  also  a  secretion.  In  the  secretion  of  this  matter  the  entire  secreting 
cell  is  destroyed  in  the  process.  In  certain  Protozoa  diffused  pigments 
arise  as  direct  products  of  alimentation.  In  Vampyrella,  for  instance, 
the  red  color  of  well-fed  specimens  is  due  to  the  color  of  the  digested 
food  held  within  the  protoplasm  of  the  cell.  M.  von  Linden  indicates 
that  the  red  pigment  of  the  intestine  of  the  larva  of  Vanessa  is  the  result 
of  a  peptic  digestion  of  the  chlorophyll  of  the  larva's  food.  In  a  series 
of  his  experiments  it  is  also  suggested  that  the  red  pigment  in  the  epi- 
dermis has  the  same  origin. 


PIGMENT   TISSUES 


271 


The  origin  of  segregated  pigment  in  the  vertebrates  has  been  much 
discussed,  and  it  is  not  yet  settled.  In  the  higher  invertebrates  there 
is  little  doubt  that  dark  pigment  granules  may  be  elaborated  by  the 
epidermal  cells  as  well  as  by  connective-tissue  cells.  Sections  of  the 
mantle  epidermis  of  the  mussel,  Mytillus,  for  instance,  show  pigment 
granules  that  have  quite  likely  been  elaborated  by  the  epidermal  cells 
under  normal  conditions.  Schiedt  has  shown  definitely  that  dark  pig- 
ment granules  develop  under  pathological  or  abnormal  conditions  within 
the  epidermal  cells  of  the  oyster.  But  even  among  the  Invertebrata 
the  connective-tissue  cells  are  the  chief  elaborators  of  pigment  granules. 
These  cells  assume  various  shapes  and,  in  some  cases,  become  very 
highly  specialized  and  are  under  the  control  of  special  nervous  impulses. 
Such  is  the  case  with  the  large  pigment  cells  or  chromatophores  of  the 
cephalopod  mollusks.  In  the  mammals,  or  at  least  in  man,  melanin 
granules  are  a  connective-tissue  product.  Ehrmann,  in  a  recent  article, 
advances  the  following  views  concerning  this  conspicuous  pigment: 
''Melanin  is  intra-cellular,  and  in  the  situations  where  it  is  present  it 
occurs  in  the  deeper  lay- 
ers of  epidermal  cells 
and  in  certain  meso- 
blastic  cells  known  as 
melanoblasts}  The  me- 
lanoblasts  are  special- 
ized connective-tissue 
cells  which  are  round, 
spindle-shaped,  or 
branching,  and  are 
peculiar  not  only  for 
containing  melanin 
granules,  but  also  for 
having  larger  nuclei 
which  stain  more  faintly 
than  those  of  ordinary 

cells.  Melanoblasts  occur  in  the  upper  layers  of  the  corium,  are  espe- 
cially noticeable  around  the  blood  vessels,  and  are  also  present  as  peculiar 
structures  in  the  interepithelial  lymph-spaces  of  deeper  portions  of  the 
epidermis.  The  substance  is  a  derivative  of  blood-pigment,  the  mate- 
rial of  which  it  is  formed  getting  out  of  the  blood  vessels  into  the  peri- 
vascular  tissue  spaces,  where  it  is  taken  up  by  the  melanoblasts  and 
transformed  into  melanin.  The  epidermal  cells  do  not  elaborate 
melanin,  but  absorb  it  from  the  melanoblasts  in  the  interepithelial 
lymphatics." 

The  connective-tissue  cells  that  have  become,  highly  specialized  as 


FIG.  234.  —  Two  figures  of  dividing  pigment  cells  from  the 
skin  of  a  larval  salamander.     (After  ZIMMERMANN.) 


272 


HISTOLOGY 


pigment  cells  divide  mitotically.     Such  mitosis  has  been  observed  by 
Flemming  and  Zimmermann  (Fig.  234). 

As  an  example  of  diffused  pigment  in  an  animal  product  we  may 
take  the  secretion  of  the  ink-sac  of  a  squid.  The  ink-sac  is  a  pear- 
shaped  organ  lying  beneath  the  integument  of  the  mantle  wall.  In 
origin  it  is  an  invagination  of  the  walls  of  the  rectum.  Its  size  varies 
with  the  size  of  the  animal ;  its  greatest  transverse  dimension  is  about 
one  eighth  the  diameter  of  the  body  at  the  level  of  the  ink-sac.  From 
the  epithelium  of  the  fundus  of  this  sac,  epithelial  pouches  are  constricted. 


FIG.  235. — A,  diagram  of  the  arrangement  of  lobules  in  the  ink-sac  of  a  squid,  Loligo  Pealii. 
x,  regions  of  lobule  generation;  d,  regions  of  discharge  of  ink  from  lobules;  B,  epithelium  of 
youngest  lobules,  no  ink  apparent ;  C,  older  epithelium,  first  appearance  of  ink  in  the  cyto- 
plasm; D,  epithelium  from  still  older  lobule;  much  ink  formed  and  stored  in  lobule;  E,  more 
ink  being  produced  by  cells  and  ready  for  discharge.  A,  X  20.  B-E,X  600. 

These,  as  they  are  crowded  toward  the  neck  of  the  sac  by  the  continued 
proliferation  of  glandular  vesicles,  increase  in  size  and  finally  rupture 
in  the  vicinity  of  the  neck  of  the  sac  (Fig.  235,  4).  The  pouch,  as  it 
leaves  the  fundus,  is  lined  with  columnar  epithelial  cells,  with  oval  nuclei. 
The  cytoplasm  is  homogeneous  and  free  from  any  pigment  (Fig.  235,  B). 
As  the  glandular  pouch  increases  in  size,  the  cells  become  higher  and 
greater  in  diameter.  At  the  distal  ends  of  the  cells,  conspicuous  pig- 
mented  drops  appear  (Fig.  235,  C).  As  the  glandular  vesicle  con- 
tinues to  grow,  the  cells  become  wider,  their  nuclei  become  more  rounded, 
while  pigment  is  elaborated  at  the  expense  of  the  cytoplasm.  The  cells 
disintegrate  distally  and  thus  pour  out  into  the  lumen  of  the  vesicle 


PIGMENT   TISSUES 


2/3 


the  pigmented  secretion  (Fig.  235,  A  and  D).  The  glandular  pouches 
eventually  burst,  and  the  elaboration  of  pigment  continues  *at  the  ex- 
pense of  the  cytoplasm  until  there  remains  but  the  nuclei  inclosed  in 
a  shallow  layer  of  cytoplasm  (Fig.  235,  A  and  E).  These  nuclei  and 
their  cytoplasm  finally  disintegrate. 

In  Vampyrella  we  have  a  beautiful  example  of  pigmentation  which 
shows  how  closely  diffused  pigment  may  be  associated  with  assimila- 
tion. This  simple  animal  is  frequently  found  living  with  cultures  of 
fresh-water,  green  Algae.  It  consists  of  a  film  of  colorless  protoplasm 
bearing  numerous  refractive  granules.  The  sheet  of  protoplasm,  when 
containing  no  food  bodies,  is  little  more  than  a  micron  thick.  When 


FIG.  236.  —  A-C,  three  stages  in  the  life-history  of  a  Vampyrella.  A,  with  freshly  ingested  green 
food  plants;  B,  food  plants  turned  to  a  reddish  brown;  C,  the  animal  departing  from  its 
encysted  state  as  three  reddish  brown  individuals.  X  725. 

food  is  ingested,  the  protoplasm  forms  a  rounded  mass  about  which 
an  encysting  membrane  is  formed.  The  animal  remains  quiet  while 
the  food  is  being  digested.  As  digestion  proceeds,  the  plants  taken  in 
as  food  change  from  green  to'  brownish  red.  The  food  decreases  in 
size  as  its  digested  parts  are  absorbed  from  the  food  vacuole.  The 
protoplasm  becomes  pigmented  by  the  assimilated  food,  so  that  when 
the  protoplasm  breaks  from  the  cyst  as  a  free  animal  or  animals,  it  is 
conspicuously  colored  brownish  red  (Fig.  236,  A,  B,  and  C). 

Metopus  furnishes  an  example  of.  segregated  pigment  within  a 
cell.  Metopus  is  a  simple,  ciliated  infusorian  found  very  frequently 
in  old  infusions.  It  will  be  readily  recognized  from  our  Figure  237. 
At  the  anterior  end  is  a  patch  colored  dark  brown  to  black.  This, 


274 


HISTOLOGY 


when  magnified  five    hundred   diameters  or  more,  is  easily  resolved 
as  a  group  of   oval  to    rounded    pigment    granules   (Fig.    237,   pg.). 

The  cells  covering 
the  surface  of  the  man- 
tle of  a  mollusk,  like 
Mytillus,  are  colum- 
nar, with  oval  nuclei 
near  their  base.  The 
distal  ends  are  naked, 
but  their  cytoplasmic 
structure  is  hidden  by 
numerous  pigment 
granules.  These  gran- 
ules become  less  nu- 
merous near  the  nuclei 
and  are  wanting  at  the 
bases  of  the  cells  (Fig. 
238). 

Connective-tissue 
cells  are  the  most  com- 
mon elaborators  of  pig- 
ment. In  many  cases 
they  become  specially 
modified  as  pigment- 
bearing  cells.  Such 
cells  are  found  in  the 
skin  and  peritoneum  of 
the  fish  Ammodytes. 
These  pigment  cells 
are  stellate  with 


cow.  v 


FIG.  237.-Individual  of  the  protozoon  Metopus.     pg.,  segre- 

gated  pigment  granules ;  MM,  nuclei;  con.  v.,  con  tractile  vacuole.  shaped  rays.      The  One 

or    two     conspicuous 

nuclei  lie  within  the  central  cell  mass.  It  is  within  this  centrally 
disposed  cytoplasm  that  the  pigment  granules  are  most  numerous 
(Fig.  239).  This  is  an  example  of  what  is  known  as  a  pigment  cell. 

In  the  last  three  examples  the  pigment  was  mostly  confined  to  a  par- 
ticular region  of  the  cell.  In  certain  hepatic  cells  this  is  not  the  case. 
The  hepatic  cells  of  Cryptobranchus  are  polygonal  with  a  well-rounded 
nucleus  inclosed  in  a  cytoplasmic  layer.  The  cytoplasm,  when  not 
highly  pigmented,  contains  many  vacuoles.  Pigment  granules  are  elabo- 
rated more  or  less  uniformly  throughout  the  cytoplasm  (Fig.  240,  B}. 
This  pigmentation  may  be  so  great  as  to  conceal  all  cytoplasmic  struc- 
ture (Fig.  240,  C). 


PIGMENT  TISSUES 


275 


thelial  cells  from  the  ex- 
ternal covering  of  the 
mantle- fold  edge  of  a 
mussel,  Mytillus.  x  1500. 


The  chromatophore  of  the  squid  is  a  complex  structure  composed  of 
a  central  pigment  cell  surrounded  by  smaller  cells,  which  send  their  slen- 
der bodies  in  a  radial  fashion  from  the  pigment 
cell  to  supporting  connecting  tissue.  These  cells 
are  contractile  and  respond  to  nervous  impulse 
brought  by  a  special  nerve  supply.  In  this  way 
the  shape  of  the  chromatophore  is  modified. 
Change  of  form  alters  the  color  of  the  chromato- 
phore. Chun  has  shown  that  this  complex  chro- 
matophore arises  from  a  single  embryonic  con- 
nective-tissue element,  which  is  an  oval  cell  as 
represented  in  Figure  241,  A. 

This  cell  is  at  first  small  and  contains  a  single  nu- 
cleus.    The  nucleus  is  eccentric,  being  placed  at  the    FlG-  23»-  —  Pigmented  ePi- 
extreme  end  of  the  cell.    This  is  apparently  so, 
because  of  the  presence  of  an  astral  figure  which 
probably  represents  a  centrosome  with  its  rays. 

As  the  cell  grows,  it  may  be  noticed  (Fig.  241,  B)  that  the  peripheral- 
cytoplasm  or  ectosarc  becomes  differentiated  from  the  remainder  of 
the  cell  by  becoming  clearer.  It  also  sends  out  sharp-pointed  pro- 
cesses, which  are  well  seen 
in  (C).  These  processes  in- 
crease much  in  length  and 
thickness,  and  when  the  cell 
is  much  larger,  some  of  them, 
usually  three  or  more  on  one 
side,  form  a  contact  with 
some  nerve  fibers  which  come 
from  a  nerve  plexus  in  the 
integument.  This  condition 
is  shown  in  Figure  242. 

The  next  feature  to  de- 
velop is  a  differentiation  of 
the  structure  to  enable  the 
cell  to  perform  its  dual  func- 
tion. These  two  functions 
are :  the  production  and  main- 
tenance of  a  pigmented  area, 
and  the  mechanical  operation 
of  that  area  by  a  muscular 
apparatus.  A  multiplication 
of  the  nucleus  takes  place ;  in  Figure  242  the  single  nucleus  has  already 
divided  into  two.  In  Figure  243  there  are  four  nuclei,  and  two  of  them 


FlG.  239.  —  Pigment  cell  from  the  peritoneum  of  the 
sa.nd-la.unce  A  mmodytes.  Fully  extended.  Arms  can 
be  retracted  until  the  cell  is  irregularly  round.  Two 
nuclei,  between  which  the  non-pigmented  centro- 
sphere  appears.  X  180. 


276 


HISTOLOGY 


have  become  modified.  They  are  found,  here,  in  the  central  area  of 
the  cell  which  is  now  a  syncytium,  and  this  area,  which  was  already 
differentiated  from  the  ectosarc,  has  become  further  separated  by  the 

formation   of   a   distinct 

blc. x^^/y*  -'v^rST^v . v-fc.  -«»!^        sac  about  its  content  of 

pigment-bearing  c  y  t  o- 
plasm.  The  sac-waU  is 
but  partly  formed  in 
Figure  242. 

The  young,  but  com- 
pleted, chromatophore  is 
seen  in  Figure  244. 
Marked  changes  have 
taken  place.  The  pig- 
ment sac  is  inclosed  by  a 
tough,  membranous  wall. 
But  one  nucleus  remains 
within  the  sac,  and  this 

FIG.  240.  —  Three  liver  cells  from  the  salamander  Crypto-  IS  larger  and  is  spetial- 
branchus.  They  show,  in  A,  B,  and  C,  three  successive  ized  to  direct  the  Dip> 
stages  of  pigmentation,  bl.c.,  blood  cell. 

ment-formmg     activities 

of  the  inclosed  cytoplasm.  The  other  nuclei  have  multiplied  by  amitosis 
and  have  migrated  outside  the  sac,  so  that  one  lies  in  the  basal  portion 
of  each  cell  process.  These  nuclei  control  the  myo-nbril-forming  activ- 
ities of  the  cytoplasm  in  the  processes  whereby  they  render  them 
contractile.  The  processes  are  now  able  to  stretch  the  central  pigment 
sac  out  until  it  becomes  very  broad  and  visible, 
allow  it  to  contract  by  its  elasticity 
and  become  almost  invisible.  The 
chromatophores  are  arranged  in  two 
or  more  different  sets,  and  all  the 
members  of  each  set  are  connected 
with  a  common  nerve  plexus.  At 
the  same  time  the  different  sets  are 
independent  of  each  other  as  to 
nervous  control,  and  so  but  one  or 
two  or  all  or  none  of  them  may  be  FlG.  24I.-A,  B,  and  c.  Three  youngest 

expanded    at    a    time.       As   each     Set         stages  in  the  development  of  a  cephalo- 

contains  a   differently  colored    pig- 
merit  in  its  pigment  sacs,  we  have 
an  explanation  of  the  rapid  changes 
of    color    which    pass   over    a    squid   or    octopus  at  different   times. 
At  one  time  it  may  be  red,  the  next  transparent,  and  presently  turn 


pod's  chromatophore.  Each  stage  shows 
a  nucleus  and  centrosphere.  The  two 
older  ones  have  put  out  processes.  (After 
C.  CHUN.) 


PIGMENT    TISSUES 


277 


brown  or  yellow.      Many  other  animals,  as  frogs  and  lizards,  change 
their  color  in  much  the  same  way. 


FlG.  242.  —  An  older  form  of  the  chromatophore,  with  two  nuclei  and  well-developed  processes, 
two  of  which  have  established  connections  with  nerve  fibers,  nv.f.     (After  C.  CHUN.) 

Technic.  — The  only  special  technic  to  be  mentioned  in  this  place 
is  the  methods  used  to 
depigment  the  various 
sorts  of  pigmented 
tissue.  This  process 
is  necessary  in  order 
that  the  structure  and 
relations  of  the  pig- 
ment cells  themselves 
may  be  determined. 
It  is  sometimes  need- 
less owing  to  a  small 
amount  of  pigment  or 
to  a  light  color  and 
transparency  of  this 
material.  It  is  always 
a  supplementary  pro- 
ceeding, and  should 
never  be  used  without 
a  proper  relation  to  the  study  of  the  unchanged  tissue.  When  much 
pigment  is  present,  the  tissue  should  be  fixed  and  hardened  first  and 


FIG.  243.  —  Still  older  chromatophore  of  a  cephalopod.  The 
nuclei  are  four  in  number  and  have  become  differentiated. 
The  central,  pigment-containing  region  is  beginning  to  appear. 
(After  C.  CHUN.) 


2/8  HISTOLOGY 

then  steeped  in  water  containing  3  per  cent  each  of  nitric  and  hydro- 
chloric acid.  Other  mixtures  may  be  used,  as  chloroform  containing 
a  drop  or  two  of  strong  nitric  acid.  Or  50  per  cent  alcohol  warmed 
and  saturated  with  picric  acid  will  depigment.  Alcohol,  glycerin,  and 


FIG.  244. — Well-developed  chromatophore  of  the  same  cephalopod.     Processes  completed  and 
extensive  connection  with  a  nerve  plexus.     Nucleus  in  base  of  each  process,     pg.s.,  pigment 
.     sac  containing  pigment  granules  and  one  nucleus.     (After  C.  CHUN.) 

water  in  equal  parts,  through  which  chlorine  gas  is  passing,  is  also  a 
good  reagent. 

The  fixation  will  itself  often  remove  the  pigment.  This  is  not  to 
be  recommended,  as  it  is  usually  associated  with  a  poor  fixation.  Well- 
fixed  tissues  nearly  always  show  the  pigment  in  good  condition. 

LITERATURE 

CHUN,  C.     "  Uber  die  Natur  und  die  Entwicklung  der  Chromatophorcn  bei  den  Cepha- 

lopoden,"  Verh.  der  Deut.  Zool.  Gesell.     Mai,  1902,  Leipzig. 
CARLTON,  F.  C.     "The  Color  Changes  in  the  Skin  of  the  Florida  Chameleon,  Anolis  caro- 

linensis,"  Proc.  Am.  Ac.  of  Arts  and  Sciences,  Vol.  39,  Nr.  10. 
FLEMMING,  W.     "Uber  die  Theilung  von  Pigmentzellen  u.  s.  w."  in  Arch.  f.  mik.  Anat., 

Band  35. 

KEPNER,  W.  A.     "Notes  on  the  Genus  Leptophrys"  Am.  Nat.,  Vol.  40,  1906. 
VON  LINDEN,  M.  G.     "Red  and  Yellow  Pigment  of  Vanessa,"  J.  R.  Mic.  Soc.,  1904, 

Nr.  i. 
VERWORN,  MAX.     "General  Physiology,"  p.  109. 


CHAPTER   XV 
STRUCTURES   OF   ALIMENTATION 

POTENTIAL  energy  is  acquired  by  animals  in  two  ways —  (a)  as  oxygen 
taken  unchanged  from  the  atmosphere,  and  (6)  as  matter  taken  as  food 
and  transformed.  The  acquiring  of  energy  and  the  accumulation  of 
material  for  growth  through  the  transformation  of  food  materials  is 
what  we  call  alimentation.  This  process  takes  place  in  every  living 
animal.  The  process  is  a  double  one,  involving  the  dissolving  or  digest- 
ing of  the  food  and  the  absorbing  of  the  food  after  it  is  digested.  The 
structures  concerned  with  alimentation  must,  therefore,  provide  cavities 
for  the  retention  of  the  food  while  it  is  being  digested,  and  surfaces  which 
can  absorb  the  food  when  once  digested.  Food  vacuoles  and  food 
cavities  retain  the  food  until  digested.  The  surfaces  of  such  vacuoles 
and  cavities  discharge  the  necessary  digestive  juices  and  afterward 
absorb  the  digested  food.  In  the  Protozoa,  where  an  individual  con- 
sists of  but  one  or  a  few  cells,  there  can  be  no  inter-cellular  food  cavity  or 
enteron;  hence  digestion  must  take  place  within  a  cell  in  food  vacuoles. 
These  unicellular  animals  secure  their  food  by  means  of  protoplasmic 
processes,  called  pseudopods,  flagella,  cilia,  and  membranelles. 

By  these  structural  devices  food,  suspended  in  some  water,  is  carried 
within  the  cell  where  the  cytoplasm  surrounds  it  on  all  sides,  forming 
a  rounded,  digestive  vacuole.  This  vacuole  is  not  a  permanent  struc- 
ture of  the  cell. 

The  size  of  the  vacuole  is  not  constant.  Its  place  of  origin  may  be 
no  fixed  region  of  the  cell,  as  in  Amceba,  Vampyrella,  and  other  naked 
Rhizopoda;  or  the  food  vacuole  may  arise  at  a  definite  region  of  the 
cell,  as  is  the  case  in  the  Infusoria  of  which  Paramcecium  is  a  represen- 
tative; nor  is  the  position  of  the  food  vacuole  once  formed  stationary; 
it  is  carried  with  the  cyclosis  of  the  cell.  The  vacuole  receives  the  digest- 
ing fluids  secreted  by  the  cell,  and  retains  the  food  until  digested.  Its 
surface  absorbs  the  digested  food.  At  the  completion  of  this  double 
process  the  vacuole  moves  to  the  surface  of  the  cell  to  discharge  the 
waste  matter  brought  in  with  the  food  and  to  disappear  as  a  feature 

279 


28O  HISTOLOGY 

of  the  cell.  This  is  the  fundamental,  structural  device  by  which  all 
intra-cellular  alimentation  is  accomplished. 

In  all  Metazoa  the  alimentary  tissue  is  differentiated  and  distinct 
from  the  other  tissues  of  the  body;  and  in  all  cases  it  is  an  epithelial 
tissue.  Despite  the  presence  of  a  distinct  alimentary  tissue,  however, 
and  until  a  definite  food  cavity  or  enteron  is  established,  pseudopods, 
flagella,  cilia,  and  food  vacuoles  are  the  only  structural,  alimentary 
features.  The  endoderm  of  the  Porifera  or  sponges  is  an  alimentary 
tissue  composed  of  a  simple  collared  epithelium,  each  cell  of  which  is 
provided  with  a  flagellum.  This  epithelium  lines  a  cavity  which  is  not 
an  efficient  place  for  digesting  fluids  to  act  upon  food,  because  of  the 
currents  of  water  that  constantly  stream  in  and  out  of  it,  and  which  would 
carry  away  the  secretions.  Food,  therefore,  is  thrown  into  the  cell 
body  of  an  endodermal  cell  by  means  of  the  flagellum,  and  alimentation 
takes  place  within  a  food  vacuole  in  a  manner  analogous  to  the  process 
used  by  the  Infusorian. 

The  food  vacuole  limits  the  size  of  the  particles  of  food  consumed  and 
is,  therefore,  not  an  highly  efficient  alimentary  structure.  With  the 
progressive  development  of  the  food  cavity  or  enteron,  the  food  vacuole 
becomes  less  frequent. 

In  the  Ccelenterata,  polyps,  and  jellyfish,  a  food  cavity  or  enteron 
is  formed  which  is  open  at  but  one  region,  and  thus  supplies  a  place  for 
digestive  fluids  to  act  upon  food  outside  of  the  cell  body.  Here  pseu- 
dopods and  cilia  attend  both  food  vacuoles  and  an  enteron.  Besides 
the  cells,  bearing  food  vacuoles,  there  are  others  which  elaborate  di- 
gestive fluids  to  be  discharged  upon  the  food  and  to  digest  it  in  the 
enteron. 

Before  the  enteron  becomes  an  open  tube,  it  is  supplied  with  a  mus- 
cular coat  which,  by  a  churning  action,  aids  the  extra-cellular  alimenta- 
tion. In  some  of  the  platyhelminthes  the  endodermal  cells  may  bear 
food  vacuoles.  The  simple  columnar  epithelium,  however,  is  an  ali- 
mentary tissue  acting  chiefly  upon  food  contained  within  the  lumen 
of  the  enteron.  Here  there  is  a  slightly  evident  differentiation  between 
cells  of  digestion  and  cells  of  absorption.  But  the  cells  so  differentiated 
are  not  assembled  to  form  two  tissues. 

In  the  Annelida  and  all  higher  forms,  the  enteron  is  made  more 
efficient  by  a  stomodaeum  and  a  proctodaeum.  With  this  advance 
in  the  formation  of  the  general  food  cavity,  there  arises  a  differentia- 
tion of  alimentary  tissues  into  digestive  and  absorptive  tissues. 

This  differentiation  together  with  the  development  of  a  separate 
internal  digestive  cavity  enables  the  organism  to  use  a  great  variety  of 
foods,  some  of  which  are  bulky  in  nature  or  hard  or  tough.  Such  food 
must  be  first  mechanically  cut  off  to  be  swallowed,  and  some  of  it  ground 


ALIMENTARY  TISSUES  28 1 

up  to  become  a  suitable  object  for  the  digestive  juices  to  act  upon.  It 
may  also  be  crushed  or  ground  after  being  partly  digested.  This  work 
is  called  mastication.  It  must  then  be  acted  upon  by  the  tissues  differ- 
entiated to  prepare  it  for  absorption.  This  process  we  shall  designate 
as  digestion;  when  digested  the  food  is  ready  for  other  tissues  whose 
surfaces  will  absorb  it.  This  is  absorption.  All  of  these  processes, 
except  perhaps  mastication,  are  differentiations  of  the  simple  alimentary 
processes  that  take  place  in  the  Protozoa  and  in  the  alimentary  epithe- 
lium of  the  sponges  and  to  be  a  degree  in  some  higher  forms. 

All  masticating  structures  may  be  considered  as  mechanical,  diges- 
tive structures.  In  all  forms  where  an  enteron  is  present,  the  muscular 
layer  of  the  alimentary  tube  must  be  considered  to  be  a  masticating  tissue 
as  well  as  a  structure  for  moving  the  food  through  the  alimentary  canal. 
As  it  becomes  more  highly  developed,  cilia  become  less  frequent.  In  the 
Coelenterata  the  muscular  wall  of  the  enteron  is  composed  of  the  layer 
of  striped  muscle  given  off  by  the  ectodermal  cells.  In  all  these  forms 
it  -is  of  very  low  development  and  probably  secondarily  concerned  with 
alimentation.  In  the  platyhelminthes  it  has  become  a  definite  layer 
of  non-striped  muscle  fibers  closely  applied  to  the  basement  membrane 
of  the  endodermal  epithelium.  It  is  here  clearly  concerned  primarily 
with  alimentation.  This  structural  feature  persists  throughout  all 
higher  forms.  The  muscular  layer  of  the  alimentary  tube  becomes  in- 
tensified in  various  regions  to  act  as  specialized  masticating  structures. 
The  "  gizzards  "  of  annelids  and  vertebrates,  and  the  stomachs  of 
mammals  and  some  crustaceans  all  present  examples  of  such  muscular 
development.  The  epithelium  of  these  regions  becomes  more  compact 
to  serve  as  protective  rather  than  secreting  or  absorbing  structures. 

In  certain  fishes  the  action  of  the  muscular  wall  of  the  stomach  is 
aided  by  calcareous  structures  laid  down  by  the  epithelium  of  the  enteron. 
The  most  common  and  most  highly  specialized  masticating  structures 
arise  from  the  surface  of  the  stomodaeum.  The  honey-stomach  of  the 
bee  and  stomach  of  the  lobster  are  the  dilated  posterior  parts  of  the 
stomodaeum  that  have  developed  chitinous  masticating  structures  which 
operate  together  with  the  special  muscular  supply  of  this  region,  The 
radula  of  a  snail  is  a  cuticle  of  chitin  formed  by  the  epithelium  of  the 
mouth  and  oesophagus.  The  teeth,  most  common  of  all  masticating 
structures,  are  but  highly  specialized  integumentary  structures.  The 
transition  from  a  scale  to  a  tooth  is  easily  seen  in  the  region  of  the  mouth 
and  on  the  jaw  of  an  elasmobranch.  • 

Certain  glands  constitute  a  second  class  of  accessory  alimentary 
tissues.  These  are  more  or  less  complex  in  their  structure  and  may  arise 
from  any  region  of  the  alimentary  canal,  but  they  always  open  into  the 
lumen  of  this  canal.  Such  accessory  structures  vary  with  the  character 


282  HISTOLOGY 

of  the  food  eaten.  The  calcium  carbonate  glands  of  the  earthworm  are 
cesophageal  glands  which  have  arisen,  as  is  held  by  some  investigators, 
to  supply  an  alkali  to  neutralize  the  great  amount  of  acid  taken  in  with 
the  food.  Many  of  the  so-called  salivary  glands  of  vertebrates  have  been 
differentiated  as  structures  to  supply  fluids  that  mechanically  aid  diges- 
tion by  furnishing  a  fluid  for  softening  and  lubricating  the  food.  These 
glands  are  known  by  the  character  of  the  secretions  given  off  as  mucous 
glands.  The  mucous  glands  are  represented  by  the  palatine,  the  dou- 
denal  (Brunner's)  glands,  and  the  glands  of  the  large  intestine.  These 
glands  vary  in  complexity,  but  all  elaborate  a  thick  mucus,  which  serves 
primarily  to  lubricate  the  food  on  its  passage  into  and  out  of  the  ali- 
mentary canal. 

The  regions  of  the  alimentary  tube,  which  are  little  more  than 
passageways  for  food  materials,  form  another  class  of  accessory  tissues. 
These  regions  are  always  tubular,  with  lumina  that  are  narrow  when  not 
functioning  and  with  highly  developed  walls.  The  muscular  coats  are 
thickened  and  supported  by  an  intensified  connective  tissue.  The 
blood  supply  in  these  regions  is  less  than  in  the  more  active  digestive 
regions  of  the  alimentary  tube.  The  epithelium  of  a  conducting  region 
is  always  heavy  and  suited  to  withstand  abrasion.  In  some  cases  a 
heavy  cuticle  of  chitin  or  other  dense  material  is  formed.  When  this 
protecting  cuticle  is  developed,  the  epithelium  is  simple  and  columnar. 
Resistance  to  abrasion  in  the  absence  of  a  cuticle  is  met  by  stratification 
of  the  lining  cells.  In  most  cases  there  are  frequent  strata  of  the  rest- 
ing cells.  In  many  cases  certain  digestive  and  accessory  glands  pour 
digestive  and  lubricating  fluids  into  these  conducting  tubules.  The 
pharynx  and  oesophagus  are  examples  of  such  structures. 

Each  cell  of  an  absorbing  tissue  must  come  into  actual  contact  with 
the  food,  and  consequently  we  find  these  epithelia  in  the  central  cavity 
of  the  digestive  tract  to  which  the  food  is  confined;  on  the  other  hand, 
as  long  as  the  digestive  tissues  have  a  canal  leading  from  them  to  the 
lumen  of  the  alimentary  tube,  they  may  retreat  as  glands  to  remote  and 
various  regions  of  the  body.  Digested  foods  vary  much  less  than  foods 
not  digested.  Because  of  these  two  facts  we  find  absorbing  cells  and 
tissues  much  less  differentiated  than  digestive  cells  and  tissues. 

Absorbing  cells  are  not  well  defined.  Indeed,  it  may  be  said  that 
all  cells  coming  in  contact  with  digested  food  may  to  a  certain  degree 
be  absorptive,  despite  any  peculiar  structure  they  may  have.  The  most 
efficient  absorbing  tissue  is  one  that  presents  the  greatest  number  of 
living  cells  in  contact  with  digested  food.  In  regions  where  we  have 
stratified  epithelium,  therefore,  we  find  the  least  efficient  absorptive 
tissues.  A  characteristic  absorbing  epithelium  is  always  a  simple  co- 
lumnar one ;  such  a  tissue  stands  exposed  to  the  lumen  of  the  enteron ; 


ALIMENTARY   TISSUES  283 

its  extension  is  increased  by  folds  and  villi  and  other  evaginations  arising 
from  the  inner  surface  of  the  enteron.  The  cells  lining  the  posterior 
half  and  more  of  the  alimentary  tube  of  invertebrates  and  those  found 
upon  the  villi  and  folds  of  the  intestine  of  vertebrates  furnish  good 
examples  of  absorptive  cells  constituting  an  absorptive  tissue. 

The  essentially  digestive  tissues  are  mostly  glandular.  We  meet 
with  such  diversity  among  them  that  we  find  ourselves  at  a  loss  for  a  mor- 
phological basis  by  which  to  classify  them.  As  all  structures  exist 
purely  for  the  purpose  of  performing  certain  functions  in  some  particular 
manner,  we  may  be  justified  in  adopting  a  physiological  basis  for  the 
classification  of  these  tissues.  The  physiological  study  of  the  digestive 
tissues  of  invertebrates  does  not  as  yet  afford  a  basis  for  an  extensive 
classification  of  these  tissues.  All  glands  belonging  to  the  alimentary 
tube  and  the  anterior  third  or  less  of  an  alimentary  tube  may  be  con- 
sidered as  digestive  tissues. 

Among  the  vetebrates  the  digestive  tissues  may  be  classified  for  our 
purpose  as:  pancreatic,  gastric,  serous,  and  hepatic.  This  physiological 
basis  holds  only  for  the  higher  vertebrates,  where  these  functions  have 
been  assigned  to  particular  tissues.  If  the  experiments  of  physiologists 
are  to  be  considered  as  final,  the  cells  of  the  villus  region  of  the  midgut 
folds  of  insects  are  both  hepatic  and  pancreatic,  and  the  hepatic  cells 
of  mollusks  are  also  pancreatic.  The  digestive  cceca  of  the  starfish 
and  Amphioxus  have  been  shown  physiologically  to  be  pancreatic, 
though  it  is  quite  probable  that  they  have  other  functions  as  well.  By 
these  conclusions  we  may  appreciate  the  fact  that  among  the  lower  ani- 
mals where  digestive  tissues  have  been  differentiated  from  absorbing 
tissues,  there  has  not  yet  arisen  a  differentiation  of  the  digestive 
tissues  into  pancreatic  and  gastric  tissues.  The  tissues  of  the  gastric 
cceca  of  the  invertebrates  are  spoken  of  as  hepato- pancreatic  tissues. 
In  the  higher  forms,  where  this  fourfold  specialization  has  been  effected, 
most  digestive  tissues  are  glandular. 

The  pancreatic  tissues  are  those  that  secrete  ferments  that  are  active 
in  an  alkaline  medium.  These  ferments  are  ptyalin,  trypsin,  and  steap- 
sin.  These  are  the  most  active  digestive  tissues.  The  pancreatic  epi- 
thelium of  mammals  resembles  somewhat  a  serous  epithelium.  These 
tissues  are  assembled  to  form  the  vertebrate  organ  called  a  pancreas. 

The  gastric  tissues  elaborate  a  ferment  that  is  active  in  an  acid 
medium.  This  ferment  is  pepsin.  These  tissues  in  mammals  are  com- 
posed of  two  kinds  of  cells :  the  ordinary  gastric  cells  that  elaborate 
the  ferment,  and  the  acid  cells  which  supply  the  acid  necessary  for  the 
action  of  the  ferment.  The  alimentary  tissue  found  in  the  gastric  glands 
of  a  mammalian  stomach  shows  these  two  kinds  of  cells.  In  a  bird  they 
are  found  in  separate  tissues. 


284  HISTOLOGY 

The  serous  tissues  of  digestion  elaborate  a  watery  secretion  that  con- 
tains the  ferment  ptyalin  or  some  other  digestive  ferment.  In  secreting 
ptyalin  they  resemble  pancreatic  tissues.  The  serous  tissues  functionally 
represent  grouped  albumen  cells  or  serocytes,  which  are  so  frequently 
encountered  isolated  among  the  alimentary  tissues  of  invertebrates. 
The  parotid  gland  and  certain  lingual  glands  at  the  base  of  the  tongue 
are  pure  serous  glands.  The  submaxillary  gland  presents  both  mucous 
and  serous  cells ;  hence  it  is  called  a  mixed  gland. 

The  hepatic  tissues  secrete  bile,  which  is  a  fluid  active  in  the  digestion 
of  fats.  This  tissue  also  has  the  power  to  elaborate  glycogen  from  cer- 
tain soluble  carbohydrates.  With  this  function  the  hepatic  tissue  be- 
comes a  storehouse  of  energy.  In  the  lower  forms  other  tissues  than 
the  gland  known  as  the  liver  may  have  this  accessory  function,  as  the 
so-called  liver  cells  found  in  the  foot  and  dorsal  mantle  region  of  the 


FlG.  245.  —  Individual  of  Paramcecium  caudatum.  Arrows  show  course  of  food  vacuoles  (f.v.). 
nu.,  nuclei;  con.v.,  contracting  vacuoles,  one  empty  and  one  full;  f.m.,  fecal  matter;  tr., 
discharged  trichocyst.  X  375. 

fresh- water  mussel.  All  vertebrates  have  hepatic  glands  that  elaborate 
glycogen. 

Certain  alimentary  tissues  have  been  differentiated  as  structures  no 
longer  directly  concerned  with  alimentation,  as  for  example  the  poison 
glands  of  certain  reptiles.  These  will  be  considered  under  another 
heading. 

Examples  of  intra-cellular  alimentary  structures.  —  Paramcecium  is 
a  very  common  protozoon  that  is  found  in  most  infusions.  It  is  a 
slipper-shaped  creature  with  a  rounded,  narrow  anterior  extremity 
and  pointed  at  the  posterior  end.  Extending  from  the  anterior  to  the 
middle  of  the  body  there  is  a  lateral  oral  groove,  which  leads  in  a  slight 
spiral  manner  to  the  gullet  at  its  posterior  end.  By  means  of  cilia 
currents  of  water  are  created,  which  bear  food  along  the  oral  groove 
into  the  gullet.  At  the  base  of  the  gullet  the  food  and  water  taken  in 
with  it  form  a  spherical  food  vacuole.  The  vacuole  becomes  too 
large  to  withstand  the  impact  of  the  water  entering  it  from  the  gullet 
and  breaks  away.  It  is  then  slowly  carried  along  with  the  cyclosis  of 


MASTICATION 


285 


the  cytoplasm.  Into  this  vacuole  the  cytoplasm  empties  digesting 
fluids,  which  act  upon  the  food.  The  digested  food  is  absorbed  by  the 
surface  of  the  vacuole,  and  the  non-digestible  parts  of  the  food  are  thrown 
out  of  the  cytoplasm  at  a  definite  part  of  the  surface  of  the  cell.  When 
the  vacuole  is  thus  emptied,  it  disappears  (Fig.  245). 


FIG.  246.  —  Part  of  the  body  of  a  sponge,  Grantia. 
mes.,  mesoderm;  eel.,  ectoderm;  end.,  endoderm; 
spi.,  spicules.  The  black  objects  in  the  round 
bases  of  some  endodermal  cells  are  food  particles. 


FIG.  247.  —  A  sectional  diagram  of  a 
water  canal  of  the  sponge  Sycon 
gelatinosum.  cil.d.c.,  ciliated  diges- 
tive cells.  (After  PARKER  and  HAS- 

WELL.) 


In  the  sponges  there  is  a  differentiation  of  the  cells  into  ectodermal, 
endodermal,  and  mesodermal  tissues.  The  endodermal  cells  line  the 
internal  cavities  and  form  the  alimentary  tissue  of  the  sponge.  These 
cells  are  collared  cells,  each  bearing  a  flagellum  (Fig.  246).  The  water, 
bearing  food,  is  brought  into  the  endodermal  canals  at  certain  places 
and  driven  out  at  other  places  by  the  actions  of  the  flagella  (Fig.  247). 


286 


HISTOLOGY 


In  this  way  the  currents  of  water  con- 
tinually sweep  by  the  alimentary  tissue, 
preventing  digestive  secretions  from  act- 
ing upon  food  that  may  lodge  outside 
of  the  alimentary  cells.  The  food  must 
therefore  be  taken  into  the  cell  bodies 
to  be  digested  within  food  vacuoles  ;  com- 
paratively large  bodies  are  taken  into 
these  food  vacuoles  (see  Fig.  246). 

An  example  of  simple  intra-cellular 
alimentary  tissue.  —  In  the  Ccelenterata 
~/nu.  ^.sec.  g.  and  Platyhelminthes  the  endoderm  has 
been  differentiated  to  perform  the  func- 
tion of  alimentation.  It  is  of  interest 
here  to  note  that  the  alimentary  tissue 
lines  a  cavity  which  is  open  at  but  one 
end.  This  affords  a  place  for  food  to 


retion  granules;  f.v.,  food  vacuoles;    rents  of  water.     Here  we  meet  with,  in 


any  degree  efficient  as  an  extra-cellular 
cavity  or  enteron.  The  cells  lining  this  cavity  are  all  of  tall  co- 
lumnar forms.  The  oval  nuclei  lie  near 
the  base  or  in  the  lower  third  of  all  the 
cells.  The  distal  ends  of  the  cells  are  ex- 
panded and  may  bear  many  vacuoles. 
There  is  here  an  interesting  differentiation 
of  the  alimentary  cells  into  two  types  of 
cells  :  the  ordinary  cells  which  we  may  call 
the  absorptive  cells,  and  the  albumen  cells 
which  elaborate  a  digestive  ferment;  hence 
we  call  them  the  digestive  cells  (Figs.  248 
and  249).  The  cytoplasm  of  the  albumen 
or  digestive  cells  is  much  denser  than  that 
of  the  absorbing  cells.  They  are  usually 
shorter  than  the  absorbing  cells.  Their 
secretion  products  are  elaborated  in  the 

form  of  spherical  bodies  at  the  proximal  FIG.  249.  —  Six  digestive  cells  f 
end.  The  nuclei  are  easily  distinguished 
from  those  of  the  connective-tissue  cells, 
but  the  nuclei  of  digestive  and  absorbing 
cells  cannot  be  distinguished.  It  is  inter- 
esting to  note  that  both  in  Hydra  and  Bdel- 


rom 

the  enteron  of  Bdellura  Candida. 
They  show  some  food  vacuoles 
and  secretion.  In  both  this  and 
the  preceding  example  the  dark 
cell  probably  secretes  a  different 
ferment  from  that  produced  by 
the  lighter  cells.  X  870. 


DIGESTIVE    TISSUES 


287 


lura  the  digestive  cells  are  most  numerous  in  the  region  of  the  opening 
of  the  enteron.  At  no  place,  however,  are  these  digestive  cells  as- 
sembled to  form  a  tissue.  Food  vacuoles  yet  function  to  a  certain 
degree.  In  certain  absorbing  cells  such  vacuoles  are  occasionally  found 
(see  Fig.  248). 


EXAMPLES  OF  ACCESSORY  DIGESTIVE  STRUCTURES 


Masticating  Structures.  Gizzard. 
a  region  of  the  alimentary  canal  in 
which  the  muscular  layer  is  most 
highly  developed.  The  layer  of  cir- 
cular muscles  lies  next  to  the  sub- 
mucosa,  and  is  much  thicker  than 
the  outer  longitudinal  layer.  The 
elements  of  this  muscular  tissue  are 
smooth,  non-striated  muscle  cells 
(see  Fig.  96).  The  epithelium  is 
composed  of  columnar  cells.  The 
cytoplasm  of  these  cells  is  finely 
granular  and  homogeneous.  The 
oval  nuclei  lie  at  the  middle  of  the 
cell.  The  basement  membrane  is 
clearly  defined.  At  their  distal  ends 
the  cells  elaborate  a  heavy  cuticle 
which  is  constantly  being  formed  as 
it  is  worn  down  by  abrasion  in 
grinding  the  food.  Many  lympho- 
cytes find  their  way  through  the 
basement  membrane  into  the  epithe- 
lium (Fig.  250). 

In  the  gizzard  of  vertebrates,  as 
represented  by  the  bird's  gizzard, 
we  find  that  a  short  portion  of  the 
digestive  tube  is  enlarged  and  pro- 
vided with  unusually  thick  muscu- 
lar walls  in  order  that  there  may 
be  grinding  power  to  triturate  the 
food.  This  is  what  has  taken  place 
in  the  worm's  gizzard,  and  the 
similarity  is  further  made  apparent 
layer  of  substance  is  placed  on  the 


— The  gizzard  of  an  earthworm  is 


\ 


m./. 


FIG.  250.  —  Transverse  section  of  part  of  the 
gizzard  wall  of  an  earthworm,  Lumbricus. 
cu.,  cuticle;  en.c.,  endoderm  cells;  cir.m.f., 
circular  muscle  fibers;  l.m.f.,  longitudinal 
muscle  fibers;  conn.t.,  connective  tissue. 
X  400. 

by  the  fact  that  a  heavy  cuticular 
internal  surface  to  protect  the  soft 


288 


HISTOLOGY 


'&: 


•m 


conn.  t. 


parts  themselves  from  the  grinding  action  of  the  stones  that  are  kept 

in  the  lumen  to  reduce  the  food  material. 

As  in  the  worm's  gizzard,  the  cuticle  is  formed  by  the  inner  layer 

of  simple  epithelium  against 
which  it  lies  (Fig.  251).  There 
is  the  difference  here,  however, 
that  the  epithelial  layer  of  the 
bird  does  not  secrete  the  lining 
directly  from  its  primary  sur- 
face as  a  solid  layer  but  from 
a  close-set  layer  of  simple,  tu- 
bular glands  into  which  it  is 
thrown.  The  protective  mate- 
rial is  produced  from  these 
glands  and  issues  from  their 
mouths  as  a  series  of  liquid  or 
semiliquid  strings,  which  spread 
out  and  fuse  with  one  another 
to  form  the  cuticle.  They  are 
speedily  hardened  superficially 
by  the  acid  that  comes  down 
with  the  food  from  the  proven- 
triculus,  and  as  fast  as  the 
inner,  wearing  surface  is  ground 
away  it  is  replaced  from  the 
other  surface  by  the  glands 
beneath. 

These  glands  dip  into  the 
submucosa  and  form  a  consid- 
erable layer.  Their  secreting 
cells  are  said  to  produce  no 
digestive  juice,  although  the 
cuticular  lining  is  thought  to 
contain  such  materials  and  is 
used  as  a  cure  for  some  forms 
of  indigestion. 

Among  the  many  very  pe- 
culiar masticating  structures 
found  in  animals  are  some  of 
the  "  gizzards "  that  may  be 
seen  in  the  anterior  part  of  the 

digestive  tract  of  fishes.     One  such  gizzard  will  be  described  in  the 

harvest  fish,  Seserinus  paru. 


FIG.  251.  —  Part  of  a  section  of  the  wall  of  the  giz- 
zard in  an  English  sparrow,  cu.,  cuticular  pro- 
tective layer;  gl.,  simple  tubular  glands  in  which 
the  cuticular  layer  is  secreted  as  a  fluid;  conn.t., 
connective  tissue;  mus.,  muscle.  X  750. 


DIGESTIVE    TISSUES 


289 


This  organ  consists,  unlike 
the  other  gizzards,  of  an  en- 
larged, muscular  portion  of  the 
cesophagus,  whose  inner  surface 
is  beset  with  tooth- like  structures 
and  invaginated  between  these 
teeth  into  the  glands.  The  epi- 
thelium is  primarily  a  stratified 
one  and  continues  to  possess  this 
characteristic  where  it  is  in- 
volved in  the  tooth  formation. 
In  its  relation  to  the  glands  it  is 
carried  down  into  them,  its  more 
distal  cells  showing  a  marked 
secretory  activity,  probably  of 
mucin.  In  the  fundus  the  outer 
cells  have  become  so  markedly 
columnar  and  so  active  as  se- 
cretory cells  that  one  has  diffi- 
culty in  deciding  that  it  is  not  a 
true,  simple,  columnar  epithe- 
lium. 

The  presence  of  a  certain 
amount  of  stratification  at  the 
base,  however,  together  with  its 
origin,  furnish  ample  evidence 
that  it  is  in  reality  a  pseudo- 
stratified  form. 

The  teeth  are  mesodermal  in 
formation,  the  epithelium  taking 
no  share  in  their  formation. 
They  arise  from  a  common  base 
which  is  a  stiff  basket  work  of 
hard  tough  fibers  that  arise  in 
the  subepithelial  connective  tis- 
sue through  the  activities  of 
some  of  its  cells.  The  shell 
thus  formed  is  elastic  enough  to 
allow  of  the  grinding  move- 
ments of  the  gizzard.  In  fact, 
it  is  not  complete  in  the  median 
line,  thus  forming  two  halves 
which  grind  upon  one  another. 


Conn,  t.  , 


FIG.  252.  — A  single  compound  tqoth  from  the 
oesophageal  gizzard  of  the  harvest  fish,  Sese- 
rinus  paru.  /.,  transverse  and  oblique  section  of 
the  deep  and  hard  reticulum,  from  which  the 
tooth  arises;  r.b.,  two  sections  of  the  basal  ring, 
from  which  the  sides  of  the  skeletal  core  (s.c.) 
arise  to  support  the  papilla;  $/>./.,  spike-shaped 
cusps  which  pass  through  the  stratified  epithe- 
lium; conn.t.,  connective  tissue  between  skeletal 
core  and  epithelium:  s.c.,  soft  core  of  connec- 
tive tissue  with  fat  cells,  blood  vessels,  and  lym- 
phatics; gl.,  glands  at  base  of  tooth,  x  80. 


290 


HISTOLOGY 


Each  tooth  (Fig.  252)  arises  from  this  basis  of  which  it  is  a  part  and 
projects  into  the  stratified  epithelial  papilla,  which  it  conforms  to  in 
shape,  but  does  not  quite  fill.  It  is  a  hollow  shell  of  basket  work,  ir- 
regularly reticular,  so  that  but  few  fenestrae  can  be  seen  in  its  sides.  It 
is  narrow  at  the  base,  with  thick  sides  that  arise  parallel  and  diverge 
as  they  pass  distally  until  they  arch  over  and  meet  at  the  top.  This 
top  is  thus  broad  and  blunt.  The  whole  structure  has  much  the  shape 
of  a  balloon  or,  still  more,  that  of  an  ordinary  incandescent  electric- 
light  bulb.  Its  walls  remain  at  a  little  distance  from  the  stratified 
epithelium,  being  kept  separate  by  a  layer  of  lax  connective  tissue.  The 
inside  of  the  tooth  is  filled  with  a  connective  tissue  that  contains  much 
lymphatic  tissue  in  its  meshes  as  well  as  an  abundant  blood  supply. 

As  so  far  described  the  structure  could  not  act  as  a  tooth  at  all,  being 
covered  by  a  thick,  stratified  epithelium.  It  possesses,  however,  on  its 
upper,  outer  surface  a  series  of  strong,  hollow  spines  which  project 
through  the  stratified  epithelium  and  come  in  contact  with  the  food. 
At  the  point  where  they  pierce  the  epithelium  the  basement  membrane 
is  reflected  distally  along  the  spines,  and  the  epithelium  sends  a  close- 
fitting  layer  a  short  distance  proximally  around  the  spike.  Thus  a 
gum  is  formed.  The  interior  of  the  spike  is  filled  with  very  active  cells, 
which  line  its  interior  and  through  whose  agency  its  walls  are  formed 
and  maintained. 

This  tooth  is  not  used  to  grind  with,  but  its  sharp  spines,  reaching  a 
short  distance  into  the  lumen  of  the  digestive  tube,  tear  and  grate  the 
flesh  of  the  animals  which  are  eaten  by  the  harvest  fish,  and  thus  prepare 
them  for  digestion.  In  some  other  fishes  there  is  a  gizzard  with  a  similar 

structure,  except  that  the  teeth, 
instead  of  lying  inside  the  gum 
with  only  the  tips  of  their  spikes 
projecting  through  its  surface, 
stand  bodily  out  from  the  gum  and 
are  hard  and  thick  with  a  white, 
polished  surface.  They  have 
shorter,  thicker  spikes  and  are 
used  for  direct  mastication. 

The  radula  of  the  snail,  Helix, 
is   composed   of   numerous  chiti- 
FIG.  3S3. -Epithelial  fold  in  which  the  radula    nous  processes  which  are  directed 

or  t-oUhed  tongue  of  Helex  pomatia  is  devel-     posteriorly. 


oped,      r.,   teeth  of  radula. 
SCHNABEL.) 


X  350.     (After 


These  are  produced 
in  a  certain  epithelial  fold  of  the 
mouth  by  a  particular  set  of  epi- 
thelial cells  (Fig.  253).  They  may  be  regarded  as  parts  of  the  cuticle 
which  these  cells  form.  As  they  are  formed  they  are  pushed  anteri- 


DIGESTIVE    TISSUES 


291 


orly  over  other  columnar  cells  to  be  exposed  on  the  radular  surface 
where  they  function  as  cutting  and  masticating  structures. 

The  honey-sac  of  a  bee  is  the  dilated  posterior  end  of  the  stomadceum. 
The  wall  of  the  sac  has  a  muscular  coat.  It  is  lined  with  a  low  columnar 
epithelium  which  secretes 
a  dense  cuticle.  At  the 
posterior  end  the  sac  is 
thrown  into  a  rounded 
prominence  which  pro- 
jects anteriorly  into  the 
lumen  of  the  sac.  The 
cuticle  over  the  curved 
surface  is  modified  to 
form  numerous  pointed 
processes,  which  are  di- 
rected towards  the  open- 
ing at  the  apex  of  the 
prominence.  This  open- 
ing leads  through  a  narrow 
passage  into  the  intestine. 
The  wall  of  this  passage  is 
highly  muscular.  Its  lu- 
men has  many  long,  cu- 
ticular setae  which  are 
directed  posteriorly  (Fig. 
254).  These  two  sets  of 
cuticular  processes  ^func-  FIG  254.  _  Longitudinal  section  of  honey,sac  of  ^ 

tlOn  as  mechanical  aids   tO          p.,   pointed    processes    on    curved    epithelial    surface; 
digestion.       By     means     Of         £,  set*  on  inner  lining  of  muscular  passage.      (From 
3  PACKARD  after  CHESHIRE.) 

them  nectar  or  pollen  is 

passed  into  the  intestine  as  demanded.  When  pollen  is  demanded,  the 
short  processes,  by  their  action,  carry  pollen  grains  with  a  certain 
amount  of  nectar  into  the  narrow  passage  leading  to  the  intestine.  As 
this  is  being  done  the  passage  is  closed  posteriorly.  A  constriction 
then  passes  anteriorly,  according  to  Cheshire,  sending  the  nectar  back 
into  the  honey-sac  and  leaving  the  pollen  grains  held  by  the  long, 
cuticular  se.tae.  These  then  pass  the  pollen  into  the  intestine  with 
little  or  no  nectar. 

Teeth.  — The  structure  of  a  tooth  is  perhaps  best  studied  in  a  series 
of  developing  teeth.  We  have  chosen  the  teeth  of  a  dogfish  for  this 
study.  In  the  embryo  dogfish  the  mouth  is  lined  with  a  stratified 
epithelium,  the  basal  cells  of  which  tend  to  be  columnar.  Along  the 
inner  margin  of  the  jaw  this  stratified  epithelium  forms  a  crescent-shaped 


2Q2 


HISTOLOGY 


groove.  The  anterior  wall  of  this  groove  becomes  a  primitive  dental 
ridge.  Over  the  surface  of  this  ridge  structures  arise  which  are  funda- 
mentally similar  to  embry- 
onic integumentary  scales. 
At  the  fundus  of  this  dental 
groove  these  structures  are 
continually  forming,  and 
travel  anteriorly  by  a  move- 
ment of  the  whole  epithelium 
towards  the  mouth  of  the 
groove.  In  its  earliest  stage 
the  tooth  appears  as  a  pa- 

FIG.  255. — The  two  earliest  stages  of  tooth-formation  ...        -              ,        .  . 

in  the  fundus  of  the  dental  groove  of  a  young  dogfish,  P"!*    formed     of  a    COre     of 

Acanthias  vulgaris.      p.,  mesodermal    papillae;    ep.,  mesenchyme  and  an  epider- 

basal  layer  of  the  stratified  epithelium,     x  80.  i     i        .-,     /T-,.  \       r^-i 

mal  sheath  (Fig.  255).    The 

columnar  cells  forming  the  lower  stratum  of  the  epidermis  enlarge  to 
become  the  enamel-forming  tissue.  The  other  layers  of  the  epidermis 
disintegrate.  In  Figure  256  remains  of  epidermal  (B,  e.p.)  cells  are 
lying  over  the  enamel  cells.  The  cells  of  this  enamel  tissue  become 
much  taller;  their  nuclei  move  towards  the  distal  ends  of  the  cells, 
and  the  cytoplasm  becomes  highly  vacuolated  at  the  proximal  ends  of 


FIG.  256. — A,  larger  developing  tooth  of  Acanthias  than  that  shown  in  Figure  255.  bl.v.,  blood 
vessels  beginning  to  enter  the  papilla.  Longitudinal,  vertical  section  X  80.  B,  enlarged 
detail  of  A  at  x  to  show  the  enamel  cells  (en.c.)  and  the  young  layer  of  enamel  (en.l.);  e.p., 
remains  of  epidermal  cells;  den.  c.,  outer  dentine  cells  of  the  papilla.  No  dentine  is  yet 
deposited.  X  400. 

the  cells.    The  hardest  part  and  outer  layer  of  the  crown,  the  enamel, 
is  elaborated  by  these  cells  and  secreted  at  their  proximal  ends.    These 


DIGESTIVE    TISSUES  293 

cells  are  destroyed  as  the  crown  of  the  tooth,  supplied  with  its  enamel, 
emerges  from  the  dental  groove. 

With  the  evolving  of  this  enamel  tissue  the  papilla  grows  and  as- 
sumes the  shape  of  a  mature  tooth  (Figs.  256  and  257).  The  mesen- 
chyme  becomes  differentiated  into  an  inner  and  an  outer  zone  of  cells. 
The  inner  zone  gives  rise  to  the  mass  of  connective-tissue  elements 
known  as  the  pulp.  This  pulp  contains  stellate  connective-tissue  cells, 
connective-tissue  fibrils,  and  a  semifluid  inter-fibrillar  substance.  The 
pulp  supports  the  nerve  and  blood  supply  of  the  tooth.  The  outer 
zone  of  mesenchymal  cells  becomes  pyriform,  with  their  small  ends  radiat- 
ing from  the  axis  of  the  tooth  (Fig.  258).  These  elaborate  at  their 
distal  ends  the  hard  part  of  the  tooth  known  as  the  dentine.  They  are 
called  odontoblasts.  Numerous  fine  canals  traverse  the  layer  of  dentine. 


FIG.  257.  —  Half-matured  tooth  of  same  animal;  blood  well  into  the  papilla,  whose  outer  edge  is 
hardening  into  the  dentine,  en. ,  enamel  laid  down  by  the  tall  columnar  epithelium,  x,  plane 
of  section  shown  in  Fig.  258.  X  55. 

Into  these  canals  or  canaliculi  the  odontoblasts  send  protoplasmic  pro- 
cesses. These  processes  take  part  in  the  calcifying  of  the  dentine.  This 
calcification  takes  place  first  at  the  periphery  where  the  processes  end. 
As  calcification  advances  the  protoplasmic  processes  of  the  odonto- 
blasts retreat,  and  the  canaliculi  are  almost  obliterated.  The  root  of 
the  dogfish's  tooth  is  not  developed  to  form  a  fang.  It  is  a  perforated 
plate  rather  than  a  tube.  It  is  composed  of  a  dentine  wall  without 
enamel  and  a  pulp  cavity.  The  nerve  and  blood  supply  enters  the 
tooth  through  this  root. 

Accessory  digestive  glands.  Calcium-carbonate  glands.  — These 
glands  occur  in  the  earthworm.  Externally  they  appear  as  three  pairs 
of  pouches  or  diverticula  from  the  side  of  the  oesophagus.  The  first 
pair  of  diverticula  lie  in  the  tenth  segment.  These  are  the  most  promi- 
nent. The  second  and  third  pair  lie  in  the  eleventh  and  twelfth  seg- 
ments respectively.  In  section  the  first  pair  is  clearly  seen  to  be  an 


294 


HISTOLOGY 


FIG.  258.  —  Details  of  last  figure 
at  *  enlarged,     en.c.,  enamel 


invagination  of  the  wall  of  the  oesophagus. 
These  first  diverticula  are  lined  with  colum- 
nar epithelium  differing  but  little  from  the 
epithelium  of  the  oesophagus.  These  sacs  are 
mere  storehouses  for  the  products  of  the 
true  glands.  The  second  and  third  pair  of 
diverticula  are  but  pairs  of  swollen  regions 
in  a  single  pair  of  glands.  Each  gland  ex- 
tends from  the  fourteenth  segment  anteri- 
orly to  the  tenth,  where  it  opens  into  one  of 
the  first  pair  of  diverticula.  Such  a  gland 
consists  of  a  number  of  flattened  tubes  with 
relatively  wide  lumina.  These  tubes  lie 
longitudinally  between  the  epithelium  of  the 
oesophagus  and  the  layer  of  circular  mus- 
cles. The  glandular  tubes  have  in  trans- 
verse section  the  outline  of  truncated  wedges; 
these  flattened  glands  lie  with  their  flattened 
sides  radiating  from  the  oesophagus.  They 
are  of  mesenchymal  origin.  Each  tube  is 
coated  with  a  membrana  propria  or  base- 
ment membrane.  The  wall  of  the  tube  is 
formed  by  a  deep  syncytial  layer.  The  cyto- 
plasm of  this  syncytium  is  alveolar  in  its 
appearance.  The  inner  margin  of  the  cyto- 


cells;  en./.,  enamel  layer;  d.c.,     plasm     varies     with     the     functional     periods. 

near  the  membrana  propria,   vary  in  size, 

shape,  and  position.  The  nuclei  when  young  are  oval  and  lie  near 
the  membrana  propria,  with  their  long  axes 
parallel  to  the  base  of  the  syncytium.  In  the 
height  of  their  development  the  nuclei  are 
spherical  and  lie  near  the  middle  or  towards 
the  margin  of  the  cytoplasm.  The  secretion  of 
lime  takes  place  near  the  margin  of  the  cyto- 
plasm. In  the  elaboration  of  lime  or  calcium 
carbonate  both  the  marginal  nuclei  and  the 
cytoplasm  disintegrate  (Fig.  259). 

Mucous  glands  from  the  base  of  the  tongue 

of  a  bat  will  serve  to  show  an  accessory  ali-  FIG.  259.  — Section  of  a  por- 
mentary  tissue  used  for  lubrication.  Each  of  tion  of  epithelium  from  the 
these  glands  is  a  branching  tubular  gland.  Its  ££X±S±±! 
cells  are  tall  columnar  elements  measuring  (After  HARRINGTON.) 


DIGESTIVE    TISSUES 


295 


about  9  by  25  microns.    Their  cytoplasm  has  been  inflated  by  a  secre- 

tion mass  so  as  to  form  a  capsule  with  but  a  film  of  cytoplasm  for 

a  wall.    The  small,  rounded  nu- 

cleus, more  or  less  distorted  by 

the  pressure  of  the  secretion  sub- 

stance, lies   at    the  base   of  the 

cell   in  the  thickest  part  of  the 

cytoplasm.    The  secretion  mass 

is  mucus.    Fresh  mucus  is  a  vis- 

cid,   cloudy    substance.     In    all 

Specimens  fixed  in  formalin,  alco-  FIG.  260.  —Part  of  the  secreting  epithelium  from 
hoi,  Or  acids  it  appears  as  a  a  mucous  gland  in  the  base  of  the  bat's  tongue. 
.  r  .,  r.c.,  resting  cell  (young  cell?),  x  700. 

dense,  floccular  mass  which  does 

not  stain  readily  in  acid  stains  (Fig.  260).  This  cell  type  is  character- 
istic of  all  mucous  tissues,  except  that  where  the  mucous  cells  are 
scattered  they  are  enlarged  distally  and  are  called  goblet  cells. 

Conductive  tissues.  —  The  oesophagus  of  the  squid  is  a  tube  with  a 
thick  wall.  The  outer  coat  is  composed  of  connective  tissue  covering 
a  heavy  layer  of  circular  smooth  muscle  fibers  inside  of  it.  Beneath 
this  is  a  longitudinal  layer  of  smooth  muscle.  A  connective-tissue  sub- 

mucosa  supports  the  in- 
ner layer,  which  is  com- 
posed of  very  low,  stout, 
columnar,  epithelial  cells. 
Each  cell  has  a  finely 
granular,  homogeneous 
cytoplasm  and  an  oval 
nucleus.  These  cells  elab- 
orate a  tough  cuticle  (Fig. 
261)  which  is  probably 
elastic.  When  not  in  use, 
the  whole  membrane  lies 
in  longitudinal  folds. 

The  oesophagus  of  the 
cat  has  also  a  thick  wall 
composed  of  two  muscu- 
lar layers,  a  connective- 
tissue  submucosa,  and  an 

_  * 

epithelium.  It      differs 

from  that  Qf   the    squ^  Jn 
.       ,. 

having  the  longitudinal 
muscles  outside  the  circular  muscles.  The  more  significant  difference, 
however,  is  in  the  epithelium.  A  cuticle  is  not  developed;  instead  the 


FIG.  261.  —  Transverse  section  of  oesophagus  of  squid, 
Loligo  Pealii.  cu.,  cuticle;  ep.,  lining  epithelium;  conn.t., 
connective  tissue;  l.mus.,  longitudinal  muscle  fibers; 
c.mus.,  circular  muscle  layer.  X  200. 


HISTOLOGY 


epithelium  is   greatly   stratified    (Fig.    262).      The   dead,   outer    cells 
resist  abrasion  as  the  cuticle  does  in  the  oesophagus  of  the  squid. 

Absorptive  tissue.  —  The 
lumen  of  the  small  intestine 
of  a  pigeon  bears  numerous 
villi.  Each  of  these  has  a 
median,  connective-tissue 
frame-work  —  the  mucosa. 
Within  the  mucosa  are  nerve 
fibers,  lymphatic  vessels,  and 
blood  vessels.  These  vessels 
are  conspicuous,  and  it  is 
into  them  that  the  assimi- 
lated products  are  emptied 
(Fig.  263).  The  villus  is  cov- 
ered with  a  columnar  epithe- 
lium. There  are  scattered 
goblet  or  mucous  cells  in  this 
epithelium.  These  are  not 

FIG.  262.  —  Part  of  a  transverse  section  of  the  stratified,  concerned    with    the    absorp- 

epithelial  lining  of  a  cat's  oesophagus,  b.m.,  basement  tjon  Qf  fOQ(J.     The  cells  most 

membrane,     x  700.  .  .  .  ,     ,. 

numerous  in  this  epithelium 

are  tall  cells  with  oval  nuclei  lying  in  the  lower  third  of  each  cell.    The 
free  or  distal  end  of  each  cell  is  marked  by  a  dense  cuticular  border 


-.fr.W. 


.leu.. 


which  cannot,  however,  be 
compared  with  a  real  cuticle. 
Lymphocytes  are  said  to 
aid  in  the  absorbing  of  fats 
as  emulsions.  Two  of  these 
cells  are  shown  lying  be- 
tween the  absorbing  cells  in 
Figure  263.  Others  travel 
through  the  epithelium  to  lie 
in  the  lumen  of  the  canal. 
These  cells  are  supposed  to 
receive  the  fatty  emulsions 
from  the  absorbing  cells,  and 
then  find  their  way  back  to 
the  lymphatic  vessels  where 
they  disintegrate  and  dis- 
charge their  contents.  These 
cells  are  not  confined  to  the  absorbing  tissues,  but  are  found  in  the 
epithelium  of  the  stomach  as  well.  They  have  a  great  affinity  for 


bl.  ca. 

FIG.  263.  —  Portion  of  a  longitudinal  section  of  a  villus 
from  the  pigeon's  duodenum,  showing  several  absorb- 
ing cells  and  two  mucous  cells,  bl.ca.,  blood  capil- 
lary with  blood  cells  inside ;  leu.,  lymphocytes.  X  870. 


DIGESTIVE    TISSUES 


297 


eosin,  and  are  best  demonstrated  by  the  use  of  this  stain  in  combina- 
tion with  others. 

Digestive  tissues. — The  digestive  tissues  in  Cerebratulus  are  not  highly 


FIG.  264.  — Portion  of  the  digestive  epithelium  of  Cerebratulus  lactatus,  showing  a  deep 
mucous  cell.     n. ,  nucleus  of  mucous  cell,     x  1 200. 


differentiated.  In  the  oesophageal  epithelium 
resented  as  are  found  in  the  intestine.  In  this 
region  there  are  two  types.  The  columnar, 
ciliated  cells  have  oval  to  rounded  nuclei 
lying  in  the  basal  third  of  the  cell.  There  is 
an  occasional  vacuole  bearing  a  secretion 
substance  found  within  these  cells  (Fig.  264). 
The  serous  cells  in  this  region  are  more  than 
twice  as  large  as  the  ciliated  cells.  They 
extend  far  below  the  basement  membrane. 
The  nuclei  are  small  and  lie  quite  near  the 
base  of  the  cells.  The  secretion  bodies  stain 
black  in  iron  haematoxylin.  In  the  intestine 
these  elements  become  much  taller,  so  that 
the  intestinal  epithelium  is  very  thick.  The 
ciliated  cells  here  are  as  tall  as  the  albumen 
cells.  Figure  265  shows  but  the  lower  third 
of  these  cells.  The  ciliated,  secreting  cells 
are  most  numerous  in  the  dorsal  region  of 
the  intestine.  Their  cytoplasm  contains  many 
spherical  secretion  particles.  The  elliptical 
nuclei  lie  near  the  base  of  the  cells  (Fig.  265, 
A).  The  albumen  cells  in  this  region  are 
scattered  and  very  slender.  Their  secretion 
particles  are  small,  round  bodies  (Fig.  265, 
C).  On  the  ventral  side  of  the  intestine  the 
albumen  cells  are  the  more  numerous.  In 


the  same  elements  are  rep- 

Iff 


FIG.  265.  —  The  bases  of  three 
long  digestive  cells  from  the  en- 
teron  of  Cerebratulus  lactatus. 
nu.,  nuclei;  b.m.,  basement 
membrane.  Cells  filled  with 
secretion  granules.  X  1 200, 


298 


HISTOLOGY 


this  region  they  have 
greatly  increased  in  diam- 
eter. The  secretion  par- 
ticles of  these  cells  are 
large  and  oval  (Fig.  265,  B}. 
The  epithelium  of  the 
intestine  of  the  hornet 
may  be  taken  as  an  ex- 
ample of  digestive  tissue 
among  the  insects.  The 
epithelium  is  thrown  into 
wavelike  folds  or  corruga- 
tions. The  columnar  cells 
vary  in  height.  They  are 
tallest  on  the  ridges  and 
smallest  in  the  grooves. 
The  fundus  of  each  groove 
forms  a  center  from  which 
new  cells  appear  to  be 
proliferated.  The  cyto- 

FIG.  266.  —  Lower  part  of  one  of  the  fold-glands  in  the  ,             .            .      ,                   , 

intestine  (ventriculus)  of  a  hornet,  Scolia  dubia.    ge.c.,  plasm  IS  retlCUlar    to  alve- 

germinal  cells;    mus.  /.,  muscle  fibers  in   longitudinal  olar.      The    nuclei    He   near 

and  transverse  section.     Upper  cells  are  absorptive,     x  the  bases  of  the  cells  (Fig. 

266). 

We  shall  take  the  gastric  gland  of  the  crayfish  as  an  example  of 
the  more  highly  specialized  digestive  tissues  of  the  Arthropoda.     This 


FIG.  267.  — -A,  transverse  section  of  the  middle  part  of  a  tubule  of  the  crayfish's  digestive  gland; 
B,  longitudinal  section  of  the  tip  of  same  showing  the  growth  point.  Many  vacuoles  shown 
with  or  without  contained  secretion  products.  X  1000. 

gland  is  composed  of  a  very  extensive  system  of  tubules.    Each  tubule  is 
lined  with  a  columnar  epithelium  which  is  furnished  with  a  membrana 


DIGESTIVE    TISSUES 


299 


FIG.  268.  —  Cells  from  the  digestive  gland  of  Mesodon 
(Helix),  cal.ph.c.,  calcium  phosphate  cell.  Others  are 
hepato- pancreatic  cells,  b.m.,  basement  membrane,  on 
which  lies  a  narrow  connective  nucleus.  X  970. 


propria,  or  basement  membrane.  The  wall  of  the  tubule  is  relatively 
thick  and  incloses  a  narrow  lumen.  In  this  case  the  cells  are  prolifer- 
ated at  the  fundus  of  the  ,  ,  . 

cal.  ph-  c- 
tubules    and    at    certain  \ 

regions    along    the    sides.  *W?" 

When  immature  they  are 

small  with  compact,  dense 

protoplasm.     The    nuclei 

are  oval  to  slightly  irregu- 
lar in   shape.    The   cells 

and  their  nuclei  increase 

in    size.     The    cytoplasm 

becomes  vacuolated  at  the 

distal  ends.    Within  these 

vacuoles     secretion,     and 

perhaps  certain  excretion, 

products  appear  (Fig.  267,  A  and  B).    When  the  cell  is  fully  grown  it  has 

attained  a  great  size,  and  bears  an  immense  vacuole  which  crowds  most 

of  the  cytoplasm  and  the  nucleus  to  the  base  or  one  side  of  the  cell  (Fig. 

267,^). 

The  general  digestive  epithelium  of  mollusks  is  strongly  ciliated 

(see  Fig.  52).  In  the  connection  with  this  ciliated  tissue  gastric  glands 
have  been  developed.  As  an  example  of 
this  more  highly  specialized  tissue  we  shall 
take  the  so-called  hepato-pancreatic  gland 
of  Mesodon.  The  chief  cells  of  this  greatly 
branched  gland  are  columnar  cells,  as 
shown  in  Figure  268.  These  cells  secrete 
a  ferment  that  aids  in  digestion.  They 
have  also  the  power  to  elaborate  glycogen. 
In  addition  to  the  chief  cells  an  occa- 
sional albumen  cell  and  as  frequently  a 
calcium  phosphate  cell  is  met  with. 

The  digestive  tissues  of  Amphioxus  are 
but  little  specialized.    The  intestinal  epi- 
thelium is  carried  into  the  hepaftic  ccecum 
or  gastric  gland.    This  epithelium  is  corn- 
ends  of  the  same  sort  of  epithelium    posed  of  very  slender  ciliated  cells.    Each 
3°°°-    cell  according  to  Schneider's  figure  bears 
a  single  cilium.    Occasional  albumen  cells 

are  found  lying  among  the  ciliated  digestive  cells.    The  nuclei  of  these 

cells  lie  farther  from  the  basement  membrane  than  do  the  nuclei  of 

the  ciliated  digestive  cells  (Fig.  269). 


FIG.  269,  A  and  B.  —  A,  digestive  epi- 
thelium from  intestine  of  Amphi- 


to  show  relations  of  cilia, 

(B  is  after  SCHNEIDER.) 


3oo 


HISTOLOGY 


sec.— —^ 


sec.  fi.-- 


Pancreatic  tissue.  —  The  pancreas  of  a  frog  is  a  compound  tubular 
gland.  The  tubules  are  incased  by  a  delicate  membrana  propria, 
and  are  held  together  by  a  connective  tissue, 
which  carries  the  nerve  and  vascular  supply. 
The  wall  of  the  tube  is  formed  by  simple 
columnar  epithelium.  The  cytoplasm  of  a 
pancreatic  cell  bears  a  variable  number  of 
fibrils,  some  of  which  are  always  found  in 
the  basal  third  of  the  cell.  The  distal  end 
of  the  cell  is  usually  granular  and  filled 
with  spherical  secretion  particles.  The  nu- 
cleus always  lies  in  the  cytoplasm  bearing 
these  fibrils.  There  is  frequently  found  in 

FIG.  270.  —  Pancreas  cell  from  sala-  .  11 

mander.  sec.,  secretion  substance;   a  pancreatic  cell  a  round,  dense  body  lying 

^..secretion fibrils.  (After MATH-   near  tne  nucleus,  called  the  paranucleus  or 

nebenkern.     This  has  been  interpreted  by 

Mathews  as  a  tangle  or  knot  of  the  fibrils  that  are  commonly  found  in 
the  pancreatic  cells  (Fig.  270). 

Gastric  tissue.  —  The  gastric  glands  of  a  muskrat  are  tubular. 
The  glands  are  supported  in  the  mucosa  of  the  stomach.  These  glands 
open  through  depressions  or  crypts  into  the  stomach.  Each  gland  is 
incased  in  a  membrana  propria.  The  glandular  epithelium  is  composed 
of  two  types  of  cells.  The  smaller  and  more  numerous  ones  are  small 
columnar  cells.  The  cytoplasm  is  rather  dense ;  the  nucleus  is  round  and 
located  near  the  center  of  the  cell.  They  much  resemble  serous  cells. 
They  receive  the  name  of  chief  cells  (Fig.  271,  A,  c.c.).  These  cells 
elaborate  a  digestive  fluid  which  is  active  in  an  acid  medium.  Crowded 
back  by  the  chief  cells  and  lying  more  remote  from  the  lumen  of  the  tube 
are  larger  cells  which  secrete  hydrochloric  acid.  These  are  known  as 
the  acid  cells.  An  acid  cell  is  usually  rather  large.  Its  original  form 
was  columnar,  but  in  most  cases  the  acid  cell  has  a  shape  conforming 
to  its  position.  The  cytoplasm  is  reticular  in  appearance  or  highly 
vacuolated,  and  is  not  so  dense  as  the  cytoplasm  of  a  chief  cell  or  a  serous 
cell.  The  nucleus  is  spherical  and  centrally  placed.  Although  lying 
remote  from  the  lumen,  there  is  always  a  passage  between  the  chief  cells 
for  the  secretions  of  the  acid  cells  to  pass  out  (Fig.  271,  A,  a.c.}.  In 
the  birds  these  two  types  of  cells  form  different  glands,  so  that  there  are 
glands  lined  with  chief  cells  and  others  with  acid  cells.  These  glands  lie 
in  different  parts  of  the  enteron.  The  acid  glands  are  always  anterior 
to  the  glands  with  the  ferment-secreting  cells.  The  latter  are  tubular 
glands  that  dip  into  the  submucosa  of  the  gizzard  (see  Fig.  251). 
The  acid  glands  are  found  in  the  proventriculus.  Each  gland  is  a  com- 
pound gland.  There  is  a  central  tube  which  is  lined  with  an  epithelium 


DIGESTIVE   TISSUES 


301 


but  little  differentiated  from  the  epithelium  lining  the  lumen  of  the  proven: 
triculus.     From  this  axial  primary  tube  many  secondary  tubes  radiate 


FlG.  271.  — A,  transverse  section  of  gastric  gland  in  stomach  of  muskrat,  Fiber  zibethkus. 
c.c.,  chief  cells;  a.c.,  acid  cells;  x,  cleft  between  chief  cells  through  which  the  secre- 
tion of  the  acid  cells  passes  into  the  lumen.  B,  parts  of  the  walls  of  two  adjoining 
acini  in  the  proventiculus  of  the  pigeon,  Columba.  The  epithelial  cells  are  acid  cells 
exclusively. 

along  its  entire  length  and  its  fundus.  The  secondary  tubes  are  structures 
with  spacious  lumina.  They  empty  into  the  axial  tube  which  serves  as  an 
excretory  crypt  or  duct  for  the  compound  gland.  The  epithelium  of 
a  glandular  tube  is  supported  by  a  membrana  propria.  It  is  composed 
of  large  acid  cells  which  are  not  generally  crowded,  hence  are  columnar. 
In  regions  where  a  cell  is  crowded  by  its  fellows  it  takes  the  triangular 
form  of  mammalian  acid  cells  (Fig.  271,  A,  a.c.). 

Serous  tissue.  Serous  tissue  of  a  gland  from  the  base  of  the  bat's 
tongue.  — This  is  a  branching  tubular  gland.  The  cells  when  quite 
active  are  greatly  distended  and  columnar  or  even  pyramidal  in  form. 
They  are  rather  small,  measuring  about  twenty 
microns  in  height  and  less  in  width.  The  cyto- 
plasm contains  numerous  small  excretion  gran- 
ules which  are  most  numerous  and  largest  at 
the  distal  ends  of  the  cell.  The  nuclei  are  oval 
to  rounded.  They  lie  near  the  center  of  the 
cytoplasm  for  the  most  part  in  the  lower  third. 

r      T  11       m    i      ,          •         .  ,1         i.  11     FIG.  272. — Acinus  of  a  serous 

of  the  cell.    Tubules    bearing   cells    distended 
with  secretion  particles  have  small  lumina  (Fig. 
272).    When  the  cells  have  given  off  their  secre- 
tion contents,  they  shrink  and  thus  form  a  large  lumen  in  the  tubule. 
Hepatic  tissues.  —  The  liver  of  Cryptobranchus  must  be  considered  a 


gland  from  the  base  of  the 
tongue  in  a  bat.  To  show 
the  serous  cells.  X  700. 


302 


HISTOLOGY 


tubular  gland.  The  branching  has  been  so  extensive  that  the  lumina 
which  constitute  the  so-called  Hie  capillaries  have  been  very  greatly 

reduced  and  branched.  These  bile 
capillaries  may  have  a  wall  of  five 
cap  or  six  surrounding  cells  or  of  but  two 
cells.  The  hepatic  cells  are  cubical 
to  polyhedral.  Their  faces  approxi- 
mate to  form  the  lumina  or  bile  capil- 
laries. The  bile  capillaries  send 
intra-cellular  branches  into  the  cyto- 
plasm of  the  cells  (Fig.  273).  The 
cytoplasm  is  seen  to  be  vacuolated 
when  free  of  glycogen  and  pigment 
granules.  The  nuclei  are  spherical 
and  lie  near  the  center  of  each  cell. 
Chromatin  is  very  conspicuous  in 
FIG.  273-- Goigi  preparation  of  a  portion  of  these  nuclei.  It  forms  large  deeply 

frog's  liver.     Cells  and   nuclei  in  outline,    staining     bodies     Suspended    Upon    a 
b.cap.,  intercellular  bile  capillary;  b.ch.    ...  .      ,  1-1  i 

intra-cellularbile  channels.     X  75°.  lmmg  reticulum  ;    a  Spherical  micleo- 

lus  is  usually  to  be  seen  within  the 

nucleus  (Fig.  274).  Canals  in  the  cytoplasm  empty  into  the  bile  cap- 
illaries. The  bile  capillaries  converge  to  form  hepatic  ducts,  which 
lead  off  from  the  lobes  of  the  liver  to  the  bile  duct. 

Technic. — These  tis- 
sues are  easy  to  handle  bl  c,— — • 
for  general  work  and 
very  few  special  processes 
are  necessary.  Owing  to 
the  presence  of  food  ma- 
terial in  the  cavities,  and 
especially  to  the  fact  that 
the  tissues  themselves 
secrete  digestive  juices 
from  which  they  are 
immune  only  as  long  as 
life  exists,  it  can  be  seen 
that  they  must  only  be 
taken  from  the  freshest 
of  newly  killed  animals 
and  must  be  immediately 
placed  in  the  fixative. 
This,  however,  often  causes  another  trouble,  the  contraction  and  shrink- 
ing away  of  the  still  living  inner  parts  from  the  already  fixed  and  stiffened 


B 


FIG.  274. — Three  liver  cells  from  the  salamander  Crypto- 
branchus.  They  show,  in  A,  B,  and  C,  three  successive 
stages  of  pigmentation,  bl.c.,  blood  cell. 


DIGESTIVE  TISSUES  303 

outer  epithelia.  This  almost  always  occurs  in  the  villi  of  the  mammal 
intestine.  It  may  be  avoided  by  several  devices,  as  the  injection  of  the 
fixative  through  the  blood  channels  after  the  use  of  amyl  nitrite. 
Also  by  the  complete  killing  of  the  tissue  by  the  use  of  chloroform. 

The  tissues  are  sometimes  very  delicate  and  susceptible  to  handling. 
These  fragile  parts  may  often  be  fixed  in  situ,  especially  by  the  use  of 
fixatives  containing  formalin.  Chrom-aceto-formol  is  one  of  the  best 
for  this  purpose.  Gentle  injections  into  the  lumen  and  into  the  sur- 
rounding cavities  should  be  made. 


LITERATURE 

Cotte,  Jules.     "Contribution  a  1' etude  de  la  nutrition  chez  les  Spongiaires,"  Bull.Scien. 

France  Belgique,  7,  38,  pp.  420-573. 
Jordan,    H.     "Die    physiologische-morphologie    der     Verdauungsorgane  bei  Aphrodite 

oculeata,"  Zeits  f.  Wiss.  Zool.,  Band  LXXVIII. 
Bizzozero,    G.      "liber    die  schlauchformigen  Driisen  des  Magendarmkanal   und  die 

Beziehungen  ihres  Epith.  zu  den  oberflachen  Epith.  der  Schleimhaut,"  Arch.  f.  mik. 

Anat.,  Band  XLII,  1893,  S.  82. 

Oppel,  A.     "  Lehrbuch  der  vergleichenden  mikroscopischen  Anatomic,"  Jena,  1900. 
Metschnikoff,  E.      "  Untersuchungen  iiber  die  intercellulare  Verdauung  bei  wirbellosen 

Tieren,"  Arbeiten  a.  d.  Zool.  Inst.  zu  Wien,  Band  V. 
Setowski,  L.     "Digestion  in  Wool-Eating  Caterpillars,"  Bull.  int.  Ac.  Sc.,  Crocovie,  1905, 

P-  535- 


CHAPTER    XVI 
THE  DUCTLESS   GLANDS 

IN  the  developing  embryos  of  vertebrate  animals  the  endodermal 
epithelium  of  the  pharynx  is  invaginated  into  a  number  of  paired  or 
median  clefts  or  pits  which,  in  most  cases,  are  later  cut  off  from  communi- 
cation with  the  digestive  lumen  by  the  constriction  and  atrophy  of  the 
connecting  ducts.  These  structures  may  be  classified  as  ductless  glands, 
although  in  some  of  them  a  duct  persists.  They  play  an  important  part 
in  the  animal's  economy,  as  can  be  readily,  although  negatively,  demon- 
strated by  experiment.  Their  structure  also  shows  that  there  must  be 
considerable  specialization  in  this  function. 

The  most  anterior  of  these  structures  is  partly  derived  from  a  pit 
in  the  dorsal  wall  of  the  mouth.  This  imagination  unites  with  a  corre- 
sponding invagination  of  the  wall  of  the  diencephalon  or  mid  brain  to 
form  the  hypophysis  or  pituitary  gland.  Further  back  are  to  be  seen  the 
five  gill  clefts  which  occur  in  all  vertebrate  embryos  as  five  invaginations 
of  the  pharyngeal  epithelium.  They  sometimes  do,  and  in  other  cases 
do  not,  open  through  the  sides  of  the  neck  to  the  exterior.  In  the  mam- 
mals they  do  not  so  open  and,  while  the  first  pair  form  no  glands,  the 
second  pair  form  the  paired  palatine  tonsils,  whose  ducts  persist;  the 
third  pair  form  another  ductless  gland  called  the  thymus  gland.  From 
a  median  invagination  of  the  epithelium  on  the  base  of  the  tongue  is 
derived  the  thyroid  gland,  and  the  fourth  and  fifth  gill  clefts  probably 
give  rise  in  the  same  way  to  the  parathyroid  bodies  or  glands. 

The  adrenal  gland  originates  in  an  entirely  different  manner  from  two 
kinds  of  tissues,  and  is  a  ductless  gland  in  the  body  cavity.  It,  too,  is 
of  vital  importance,  as  is  attested  by  the  death  of  the  subject  from  which 
the  adrenals  have  been  removed.  The  carotid  gland  and  coccygeal  gland 
are  two  small  ductless  glands  found  on  the  carotid  artery,  and  ventrad 
of  the  middle  sacral  artery  respectively,  in  man  and  some  of  the  mam- 
mals. 

All  these  glands  discharge  their  secretions  into  the  blood  as  well  as 
take  their  food  materials  from  it.  They  therefore  have  a  very  incom- 
plete lumen  which  rarely  remains  in  communication  with  the  exterior 
(as  in  the  tonsils)  and  is  usually  cut  off  from  it  (as  in  the  thyroid,  thymus, 

304 


THE  DUCTLESS   GLANDS  305 

etc.),  or  they  have  no  lumen  at  all  (as  in  the  adrenal,  etc.)-  We  shall 
take  up  the  descriptions  of  these  tissues  in  the  order  in  which  they  are 
mentioned  above. 

Several  important  secreting  tissues  are  associated,  in  the  vertebrates, 
with  the  walls  of  the  brain.  In  earliest  embryonic  life  the  lower  brain 
wall  is  invaginated  to  form  a  longer  or  shorter  ventral  depression,  the 
infundibulum,  whose  lower  end  becomes  cut  off  to  form  the  hypophysis. 
The  posterior  wall  of  this  infundibular  invagination  remains  thin, 
and  becomes  of  a  complicated  sac-shaped  form,  in  some  animals 
with  the  secretory  epithelial  layer  lining  a  lumen  that  remains  in  connec- 
tion with  the  brain  cavity,  while  the  proximal  surface  of  this  epithelium 
is  brought  into  contact  with  an  abundant  blood  supply  in  sinusoids. 
This  structure  is  called  the  infundibular  gland  or,  less  properly,  the  "sac- 
cus  vasculosus." 

An  invagination  of  the  roof  of  the  oral  cavity  of  the  embryo  arises, 
and  reaches  up  until  its  fundus  comes  into  a  close  relation  with  the 
hypophysis.  The  fundus  of  this  invagination  becomes  cut  off  and  de- 
velops into  a  mass  of  cellular  cords,  being  known  as  the  glandular  lobe 
of  the  hypophysis. 

The  neural  lobe  of  the  hypophysis  generally  consists  of  a  fibrous 
medulla  and  a  cortical  region  of  cells  which  may  have  some  glandular 
function,  although  none  has  been  actually  determined. 

We  shall  first  pay  particular  attention  to  the  infundibular  gland 
as  found  in  the  lower  vertebrates  and  then  to  the  glandular  lobe  of  the 
hypophysis  as  seen  in  one  of  the  higher  forms. 

The  infundibular  gland  of  a  flounder,  Pseudopleuronectes  Americanus, 
begins  in  the  young  embryo  as  a  sac-like  invagination  of  the  posterior 
wall  of  the  infundibulum.  The  walls  of  this  sac,  therefore,  consist  of 
an  epithelium  which  was  previously  invaginated  from  the  ectoderm 
to  form  the  brain  and  would  otherwise  have  become  nervous  in  function. 
This  sac-like  invagination  becomes  folded  and  bent  by  many  septa  that 
arise  from  its  walls.  With  each  fold  there  comes  a  blood-vessel  loop 
which  enlarges  to  become  a  thin-walled  sinusoid.  The  whole  number 
of  vessels  soon  form  a  plexus  that  carries  a  large  quantity  of  blood  and 
gives  the  structure  its  characteristic  red  color  during  life. 

Figure  275  shows  parts  of  two  rows  of  the  secretory  cells  and  parts  of 
the  blood  spaces  upon  which  they  lie.  These  latter  are  filled  with  the 
typical  nucleated  corpuscles  of  the  fishes.  The  cells  are  large  and  heavy 
with  a  dense  cytoplasm  that  shows  no  vacuoles  or  granules,  or  other 
evidences  of  activity.  The  nuclei  are  round,  and  placed  slightly  toward 
the  proximal  end  of  the  cell.  The  nucleolus  lies  almost  exactly  in  the 
center  of  the  nucleus,  and  the  chromatin  is  arranged  in  strands  which 
radiate  with  some  regularity  from  the  nucleolus  to  the  nuclear  mem- 


306 


HISTOLOGY 


M.' 


brane.    The  distal  end  of  such  a  secreting  cell  ends  in  a  knob-like 
process  directed  into  the  gland  lumen  (which  is  part  of  cavity  openly 

connected  with  the 
brain  lumen)  and 
a  number  of  pecul- 
iar, heavy  gran- 
ules, some  of  which 
seem  hollow,  are 
collected  around 
this  knob  like  a 
sort  of  cap. 

A  rather  re- 
markable feature  is 
the  number  of 
slightly  smaller  nu- 
clei that  lie  in  this 
epithelium  near  the 

FIG.  275.— Part  of  a  section  through  the  infundibular  gland  of  a  distal  ends  of  the 
flounder,  Pseudopleuronectes  Americanus.  lu.,  lumen;  bl.,  blood  ppllc  Thev  RDDPar 
vessels;  sec.ep.,  secreting  epithelium.  X  750.  *  ^^ 

to  be  in  the  secre- 
tory cells  or  between  them.  Well-preserved  preparations  show  that 
they  belong  to  smaller  cells  which  lie  between  the  ends  of  the  secre- 
tory cells  and  seem  to  be  degenerating  in  the  mature  gland.  No  evi- 
dences of  any  renewal  process'  can  be  seen  that  would  indicate  such 
a  casting  off  of  cells,  however. 

This  gland  evidently  secretes  some  substance  into  the  brain-cavity 
fluids.  It  is  not  the  only  point,  however,  at  which  the  brain-wall  cells 
have  given  up  their  nervous  functions  to  become  gland  cells.  At  other 
points  in  the  brains  of  all  vertebrates  are  found  places  where  the  wall 
has  been  evaginated  into  long,  branching,  and  anastomosing  tubes  of 
simple  epithelium,  and  blood  vessels  have  followed  these  tubes  in  and 
occupy  their  centers.  Thus  again  the  blood  is  brought  into  relation 
with  the  brain-cavity  fluids  with  only  a  simple  epithelium  and  the 
thin  blood-vessel  endothelium  lying  between  them.  It  is  considered 
in  this  case  that  the  epithelial  cells  are  active  in  removing  substances 
from  the  brain-cavity  fluids.  Such  a  structure  is  known  as  a  choroid 
plexus. 

The  glandular  lobe  of  the  cat's  hypophysis  will  next  claim  our  atten- 
tion. As  was  said  above,  this  is  an  epithelial  invagination  of  the  mouth 
cavity  which  has  become  subsequently  cut  off,  and  superficially  attached 
to  the  neural  lobe  of  the  hypophysis.  It  undergoes  a  complex  develop- 
ment in  many  animals,  and  comes  to  be  made  up  of  lobules  like  the 
thyroid  gland  (see  below).  In  the  cat,  however,  the  lumen  almost  dis- 


THE  DUCTLESS   GLANDS 


307 


eft.  c. 


the  glandular  lobe  of  a  cat's  hypophysis. 
c.c.,  chief  cells;  ch.c.,  chromophilic  cells. 
X875. 


appears,  and  the  tissue  (Fig.  276)  appears  as  a  series  of  compact  cords 
of  cells,  among  which  pass  many  small  blood  vessels.  This  blood  sup- 
ply is  not  as  abundant  as  it  was  in 
the  infundibular  gland. 

A  principal  point  to  be  observed 
is  that  the  cells  are  of  two  kinds,  a 
larger  cell  filled  with  a  purely  granu- 
lar secretion,  and  a  smaller  cell  that 
does  not  stain  in  the  same  way  and 
is  not  filled  with  a  secretion.  These 
may  be  two  varieties  of  cells,  or  they 
may  be  different  physiological  stages 
of  the  same  kind  of  gland  cell.  They 
have  been  designated  by  the  manner 
in  which  they  take  up  different  ani- 

lin  dyes  as  the  chief  cells  and  the    FlG.  276._Smaii  portion  of  tissue  from 
chromophilic  cells. 

The  palatine  tonsils  of  an  oppos- 
sum,  Didelphys,  may  be  considered 
here  because  of  their  origin,  and  also  because  they  act,  in  a  manner  at 
least,  as  ductless  glands,  although  the  imaginations  from  which  they  are 
derived  remain  in  open  communication  with  the  digestive  lumen.  Each 
tonsil  consists  of  a  slightly  raised  area  from  which  several  crypts  have 
been  formed  by  invagination.  The  stratified  epithelium  of  the  oral 
cavity  is  continuous  through  these  amplifications,  but  becomes  thinner 
at  the  bottom  of  each  infolding. 

Beneath  the  epithelium  is  a  thick,  mesoderinal  layer  composed  of 
connective  tissue  infiltrated  by  lymphatic  cells.  Lymph  nodules  or 
germinal  centers  are  found  through  this  mass  at  regular  intervals,  and 
they  are  typical  of  lymphatic  tissue  in  every  way.  As  a  rule,  the  lym- 
phatic tissue  is  but  one  nodule  thick.  In  many  forms,  however,  it  is 
apparently  thicker,  owing  to  a  greater  involution  of  the  epithelium. 

It  is  in  the  relation  of  this  lymphatic  tissue  to  the  invaginated  epithe- 
lium that  the  tonsil  possesses  a  specific  character  which  allows  one  to 
speak  of  it  as  a  "gland."  Some  of  the  amoeboid  lymph  cells  are  con- 
stantly forcing  their  way  distally  between  the  epithelial  celts, 'and  find- 
ing their  way  out  of  the  body  into  the  digestive  tube.  They  are  nu- 
merous in  the  saliva  and  mucus,  and  possibly  act  as  scavengers  and 
destroyers  of  bacteria.  They  pass  through  the  epithelium  in  greater 
numbers  during  certain  conditions  of  the  body,  and  they  also  come 
through  certain  parts  more  than  they  do  through  others. 

The  illustration  (Fig.  277,  A  and  B)  shows  two  small  portions  of 
the  epithelium  which  lines  one  of  the  several  tonsilar  crypts  of  the  opos- 


308 


HISTOLOGY 


sum.     A  small  amount  of  the  underlying  lymphatic  tissue  appears, 
and  lymph  cells  are  seen  passing  through  in  greater  numbers  in  the  right- 
hand  figure.   They  almost 

A  //&£&ti\  B      .          take  away  the  character- 

istic appearance  of  the 
epithelium  at  this  point, 
making  it  look  like  lym- 
phatic tissue  instead.  A 
thin,  distal  layer  of  strati- 
fied cells  persists  in  nearly 
all  stages,  being  renewed, 
when  the  leucocytes  have 
destroyed  it  by  passing 
through  in  large  num- 
bers. 

There  are  many  other 
places  in  the  digestive 
tract  in  which  lymphatic 
tissue  is  put  in  this  rela- 
tion with  the  enteric 
lumen.  Sometimes  a  sim- 
ple epithelium  forms  the 
surface  through  which  the 
leucocytes  break.  Where 
there  is  but  little  of  the 
adenoid  tissue,  there  are 
no  lymph  nodules  to  pro- 
pagate a  new  supply  of 
the  lymph  cells,  and  these 
are  brought  or  wander  in 
from  other  germinal  cen- 
ters instead.  Amoeboid  cells  resembling  leucocytes  are  always  to  be 
seen  between  the  columnar  cells  which  line  the  larger  part  of  the 
enteron.  At  other  places  the  lymph  tissue  is  collected  locally  into  iso- 
lated masses  which,  when  large  enough,  are  found  to  contain  a  germinal 
center,  and  here  the  amoeboid  cells  break  through  the  epithelium  en 
masse.  Such  are  the  Peyer's  patches  of  the  small  intestine. 

Another  pair  of  embryonic  gill  clefts,  the  third,  invaginate  as  did  the 
tonsil  glands  to  form  another,  and  temporarily  larger,  gland  which  is 
called  the  thymus  gland.  It  is  a  much  more  specialized  tissue  than  the 
tonsil,  and  grows  rapidly  until  a  rather  early  period  in  life,  after  which 
it  undergoes  a  retrograde  development,  and  finally  degenerates  in  age 
by  a  fatty  transformation.  Its  structure  and  development  are  difficult 


FIG.  277.  —  A  and  B,  epithelium  from  two  regions  in  the 
tonsilar  cleft  of  a  young  opossum,  d.s.,  distal  surface  of 
the  stratified  epithelium ;  b.m.,  basal  membrane  of  epithe- 
lium; lu.,  leucocytes  crawling  through  epithelium  and  in 
lumen;  lym.t.,  lymphatic  tissue  beneath  the  epithelium. 
X  650. 


THE  DUCTLESS   GLANDS 


309 


to  follow,  and  have  been  the  subject  of  much  study  and  controversy. 
The  two  most  distinctly  formulated  views  of  its  origin  are,  first,  that  it 
is  an  epithelial  invagination  to  which  has  been  added  a  mesodermal 
reticulum  in  which  many  leucocytes  and  other  more  specialized  cells 
have  emigrated;  and  secondly,  that  it  is  a  purely  epithelial  structure, 
some  of  whose  rapidly  multiplying  epithelial  cells  have  formed  a  reticulum, 
while  others  have  become  specialized  into  several  kinds  of  cells,  the  larger 
number  of  which  closely  resemble  leucocytes.  Many  adherents  of  this 
view  approach  a  mean  by  saying  that  true  leucocytes  of  mesodermal 
origin  do  move  in  among  the  thymus  cells,  especially  in  the  outer  or 
cortical  part.  They  sometimes  carry  this  idea  to  the  extent  of  a  third 
theory,  which  claims  that  the  cortex  only  of  the  lobules  is  mesodermal 
in  origin. 

Upon  examining  the  thymus  gland  of  a  cat  at  about  the  time  of  birth, 
we  find  the  gland  is  composed  of  a  fairly  large  number  of  solid,  cellular 
lobules  which  each  ex- 
hibit a  rather  weakly 
defined  cortical  and  a 
medullary  region.  These 
lobules  are  packed 
closely  together  to  form 
an  elongated  mass  of  tis- 
sue, and  the  medullary 
portion  of  each,  on  the 
inner  side  of  the  lobule, 
is  continued  through  the 
cortex  as  a  cord  of  tis- 
sue which  emerges  to 
unite  with  the  similar 
cords  from  other  lobules 
ani  thus  form  a  central 
connecting  mass  similar 
to  the  medulla  of  each 
lobe.  Light  coverings  of  connective  tissue  surround  each  lobule  and 
carry  a  heavy  blood  supply  into  its  tissue. 

Under  a  high  power  (Fig.  278)  the  gland  appears  to  be  a  reticulum 
of  connective- tissue  cells  in  whose  meshes  there  lie  very  many  lymph 
cells  and  other  cells.  As  shown  by  Hammar,  this  reticulum  originated 
from  a  lumenless  and  ductless,  invaginated  mass  of  epithelial  cells  from 
the  region  of  the  third  embryonic  gill  cleft.  This  mass  became  reticular 
by  the  formation  of  processes,  from  the  cytoplasm  of  its  cells,  which  re- 
mained united  with  the  processes  from  other  epithelial  cells.  By  the 
lengthening  of  the  connecting  strands  thus  formed  a.  fairly  wide-meshed 
reticulum  is  produced. 


-rtf.  '# 


FIG.  278. — Small  central  portion  of  a  lobe  of  the  thymus 
gland  of  a  kitten.  Has.b.,  HassalPs  body;  ret.c.,  reticular 
cells;  th.lu.,  thymic  leucocytes.  X  800. 


3IO  HISTOLOGY 

This  reticulum  is  the  basis  of  structure  in  the  thymus  tissue  of  young 
animals,  and  in  its  meshes  lie  the  other  cells.  These  are  the  thymic 
lymphocytes,  the  myoid  cells,  the  HassalVs  cells,  and  the  several  types  of 
ciliated  or  bordered  cells  found  more  commonly  in  the  thymus  of  lower 
vertebrates  but  also  seen  in  the  mammals. 

The  thymic  lymphocytes  -are  of  several  varieties  and  least  is  known  of 
them,  in  regard  to  their  origin  and  function,  on  account  of  the  difficulty 
of  study  which  they  possess  in  common  with  other  lymphoid  tissues, 
the  movements  of  their  cells  which  cannot  be  followed  in  life  but  must 
be  studied  by  successive  stages  in  a  series  of  different  fixed  sections. 

The  myoid  cells  are  evidently  derived  by  a  direct  specialization  of 
some  of  the  elements  of  the  epithelial  reticulum.  These  increase  in  size 
and  the  cytoplasm  shows  a  distinct  fibrillation.  The  fibrils  are  parallel 


cil.  8. 


FIG.  279.  —  A,  portion  of  thymus  tissue  from  an  infant.     Shows  a  ciliated  space  (cil.s.)     (After 
HAMMAR.)     B,  myoid  cell  from  thymus  of  a  small  pickerel  frog.     X  850. 


and  the  groups  form  circular  or  whirlpool  masses  which  center  after  a 
fashion  around  the  nucleus.  These  fibrils  show  a  muscle-like  segmen- 
tation in  some  animals,  as  the  frog  (Fig.  279,  B).  The  change  may  stop 
here  or  may  go  on  to  the  development  of  long  strands  of  cytoplasm  in 
which  the  fibrils  run  parallel  for  some  distance  and  show  a  strong  seg- 
mentation into  anisotropic  and  isotropic  segments  that  correspond  to 
those  of  muscle. 

Certain  of  the  cells  of  the  epithelial  reticulum,  situated  always  in 
the  medullary  substance,  become  enlarged  and  grouped  into  concentri- 
cally arranged  masses  which  are  known  as  "Hassall's  bodies"  from  their 
discoverer  (Fig.  278).  At  first  small  and  solid,  these  bodies  increase 
in  size  and  the  central  portion  degenerates  and  breaks  down.  The  sev- 
eral concentrically  arranged  and  stratified  peripheral  layers  show  many 
characteristics  of  a  stratified  epithelium,  and  it  was  considered  by  many 
histologists  that  these  hollow  bodies  were  the  remnants  of  the  epithelium 


THE  DUCTLESS   GLANDS  311 

which  lined  the  original  invagination  that  produced  the  thymus.  This 
view  is  incorrect,  in  so  far  as  the  original  invagination  was  not  hollow, 
but  solid,  and  therefore  had  no  lining  stratified  epithelium.  On  the 
other  hand,  this  original  invagination  was  composed  of  what  would 
otherwise  have  become  stratified  cells,  and  as  some  of  these  cells  in  the 
central  position  give  rise  to  the  HassalPs  cells,  it  might  be  argued  that 
these  bodies  represented  a  late  and  imperfect  attempt  on  the  art  of  the 
gland  to  develop  a  lumen  lined  by  a  stratified  epithelium.  The  process 
might  be  compared  with  other  invaginations  which  only  develop  their 
lumen  after  their  structure  is  well  under  way,  as  in  the  invaginated  ner- 
vous tube  of  some  teleost  fishes. 

Another  remarkable  development  derived  from  the  epithelial  reticu- 
lum  of  the  thymus  gland  is  seen  in  the  form  of  small  openings  in  single 
cells,  or  formed  by  groups  of  cells,  some  of  which  develop  cilia  or  cuticular 
edges  on  such  of  their  surfaces  as  bound  these  openings  (Fig.  279,  A). 
The  cilia  are  well  formed  and  must  undoubtedly  be  active  during  life. 
The  cavities  in  which  they  work  are  closed  and  often  contain  irregular 
masses  of  some  unknown  secretion  product.  These  ciliated  openings 
apparently  have  no  function  in  which  ciliary  motion  can  bear  a  necessary 
part,  and  they  may  be  looked  upon  in  the  same  light  as  Hassall's  bodies, 
as  vestigial  lumina  which  are  lined  with  ciliated  epithelium  instead  of 
stratified.  The  few  well-defined  mucous  cells  found  in  the  tissue  must 
also  be  regarded  in  the  same  light  until  some  more  definite  function  can 
be  proved  to  require  them. 

The  remaining  and  largest  number  of  cells  of  this  tissue  are  the  ap- 
parently amceboid  cells  that  resemble  leucocytes  of  several  varieties. 
That  some  of  these  are  derived  from  the  mesoderm  is  undoubted.  That 
any  of  them  are  transformed  epithelial  elements  is  very  improbable. 
They  have  probably  been  acquired  by  the  moving  in  of  real  lymph  cells. 
Their  slight  differences  in  structure  and  staining  reactions  are  partly 
responsible  for  the  difference  in  texture  between  the  cortex  and  medulla 
of  the  lobules. 

We  next  shall  examine  the  thyroid  gland  of  a  fish,  Raja  Icevis,  in 
which  the  organ  is  typically  developed. 

This  organ  consists  of  a  series  of  lobules  lying  side  by  side  in  a  mass 
of  vascular  connective  tissue.  Each  of  these  lobules  is  hollow  and  is 
lined  by  a  simple  epithelium  which  is  cuboidal  in  the  smaller  ones,  and 
high  enough  to  be  called  columnar  in  others.  The  nuclei  are  round  and 
full  and  lie  slightly  proximad  of  the  cell  in  the  smaller  lobules  and  well 
toward  the  basement  membrane  in  the  larger  ones  (Fig.  280).  The 
cytoplasm  is  clear  and  shows  several  sorts  of  granules  that  probably 
represent  different  stages  in  the  elaboration  of  the  secretion.  The  cells 
have  been  divided  into  two  classes  on  account  of  constant  differences 


312 


HISTOLOGY 


sec.-' 


in    the    presence    of   one    or    the    other   of  these   different   kinds   of 

granules.  The  cells  pass  the 
secretion  into  the  lumen  of 
the  lobule,  which  has  no  duct 
or  means  of  discharging  the 
mass  externally.  It  is  retained 
as  a  colloid  substance  in  the 
lumen  of  the  lobule.  There  is 
a  constant  growth  of  the 
younger  lobules,  which  appear 
at  certain  germinal  centers 
and  slowly  increase  in  size. 
Figure  280  shows  several  of 
the  cells  which  line  one  of  the 
larger  lobules  in  this  fish. 

The  thyroid  gland  is  usually 
accompanied  by  another  struc- 
ture, the  parathyroid  gland  or 
body.  In  most  of  the  higher 
vertebrates  the  parathyroid  ap- 
pears as  several  small  irregu- 
lar bodies,  the  lobes,  which  lie 
on  the  posterior  edge  of  the 

thyroid.     They  vary  in  form  and 'position,  and  may  be  two  or  four  in 

number      according 

as   one   or  both  of 

the  last  two  pairs  of 

gill  clefts  took  part 

in   their    formation. 

Each    lobe    of   this 

tissue     is     invested 

with  a  dense  connec- 
tive-tissue     sheath, 

and  its  interior  retic- 

ulum  of  loose  con- 
nective tissue  is  filled 

with   two    kinds   of 

cells   which    appear 

to  be  specific  to  the 

gland.     The     most 

abundant    of    these 

are  called  the  prin- 
cipal cells.     They  are  slightly  irregular  ovals  or  spheres,  with  a  large 


FIG.  280. — Vertical  section  of  a  small  part  of  the 
epithelium  which  lines  one  of  the  lobules  of  the 
thyroid  gland  of  a  skate,  sec.,  secretion  in  lumen ; 
bl.v.,  blood  vessel  in  connective  tissue  that  sur- 
rounds the  lobule.  X  1 100. 


FIG.  281.  —  Bit  of  tissue  from  the  parathyroid  gland  of  a  young 
cat.  Two  blood  vessels  are  seen,  near  one  of  which  is  a  single 
acidophile  cell  (ac.c.).  X  750. 


THE  DUCTLESS   GLANDS  313 

round  nucleus  whose  chromatin  forms  a  peculiar  pattern.  These  cells 
form  various  cords,  groups,  or  even  acini  (Fig.  281).  In  some  of 
them  the  cells  appear  to  be  surrounding  a  small  lumen  that  contains  a 
colloid  substance  like  the  thyroid  substances.  Besides  these  principal 
cells  there  is  another  kind  of  much  the  same  character,  but  containing 
a  smaller  and  deeper  staining  nucleus.  This  stains  in  acid  dyes  best,  and 
the  cells  are  known  as  the  acidophile  cells. 

The  blood  supply  of  this  tissue  is  very  rich.  It  consists  of  an  arterial 
system  which  percolates  through  the  tissue  in  large,  thin-walled  vessels 
which  empty  into  veins  to  return  to  the  central,  circulatory  organ.  Be- 
tween these  blood  vessels  and  the  gland  cells  are  found  but  few  strands 
of  connective  tissue.  The  vessels  are  lined  by  a  single,  thin  endothelial 
layer. 

Two  important  secretory  tissues  are  found,  in  the  vertebrate  body, 
which  pass  their  secretion  into  the  blood,  where  it  is  of  importance  to  the 
organism's  economy.  Their  constant  presence  in  the  neighborhood  of 
the  kidneys  has  given  them  the  general  designation  of  renal  bodies. 

One  of  these  bodies  is  composed  of  cells  that  originated  by  a  devel- 
opment from  cells  that  otherwise  would  seem  destined  to  become  sym- 
pathetic nerve  cells.  Instead  of  acquiring  nerve  processes  and  neuro- 
fibrils  as  well  as  perceptoryand  motor  end-organs,  they  acquire  a  secretory 
power  and  (to  the  eye)  a  peculiar  texture  which  can  best  be  noticed  by 
placing  them  in  chromium  salts,  which  they  take  up  more  readily  than 
other  cells,  and  which  stains  them  a  dark  brown. 

The  secretory  power  results  in  the  production  of  an  organic  substance 
called  "adrenaline,"  which,  when  injected  into  the  circulation  of  the  same 
or  other  vertebrate  animals,  causes  a  contraction  of  the  blood  vessels 
and  a  consequent  rise  in  blood  pressure.  These  cells  are  most  commonly 
called  the  chromaffine  cells,  and  they  form  the  paraganglionic  bodies, 
as  this  sort  of  tissue  has  been  called.  Chromaffine  cells  may  be  devel- 
oped in  other  situations  than  in  the  renal  bodies,  and  perhaps  from  other 
cells  than  young  nerve  cells.  The  function  of  such  cells  would  appear 
to  be  the  same.  Many  isolated  chromaffine  cells  also  appear  in  other- 
wise purely  nervous,  sympathetic  ganglia. 

The  second  sort  of  tissue  which  takes  part  in  forming  some  renal 
bodies  is  not  so  well  known,  and  we  shall  use  the  name  by  which  it  has 
been  most  known,  the  cortical  tissue  of  the  renal  bodies. 

This  tissue  is  composed,  as  a  rule,  of  smaller  cells  arranged  in  cords 
which  lie  between  a  series  of  anastomosing  blood  vessels.  The  cells 
lie  side  by  side  in  the  cords,  and  form  approximately  rows  which  resem- 
ble true  glands  without  a  lumen.  They  probably  secrete  some  substance 
into  the  blood. 

These  cortex  cells  are  derived  from  some  of  the  surrounding  meso- 


3H 


HISTOLOGY 


dermal  tissue,  and  they,  as  well  as  the  chromaffine  cells,  are  sometimes 
found  in  widely  different  positions.  The  presence  of  bodies  that  cor- 
respond to  renal  bodies  has  been  suspected  in  some  invertebrates.  This 
fact  is  far  from  being  demonstrated,  however,  and  the  first  of  a  very 
interesting  taxonomic  series  of  these  tissues  that  we  shall  study  are  the 
renal  bodies  of  an  elasmobranch  fish,  Raja  maculata.  Both  kinds  of 
renal  tissue,  chromaffine  and  cortical,  are  found  here,  the  former  as  a 
double  row  of  bodies,  one  row  on  the  ventral  median  edge  of  each  kidney. 
They  are  larger  toward  the  head,  where  they  form  the  "axillary  hearts." 
All  the  parts  of  these  bodies  are  in  intimate  contact  with  branches  of  the 
large  dorsal  blood  vessels. 

A  section  of  the  anterior  "axillary  heart"  shows  that  it  is  a  body  com- 
posed of  both  sympathetic  nerve  cells  and  the  characteristic  chromaffine 


FIG.  282. — A,  part  of  a  section  of  a  paired  adrenal  body  from  the  skate,  Raja  clavata;  B,  part 
of  a  section  of  the  interrenal  body  from  the  same  fish.     (After  SWALE  VINCENT.)     X  450. 


cells.  These  latter  form  the  greater  part  of  the  posterior  and  internal 
part  of  the  "  gland."  They  are  massed  around  a  blood  vessel  as  a  zone 
of  large  irregular  and  branching  cells  which  vary  much  in  size.  At  the 
surface  they  show  an  irregular  columnar  arrangement  due  to  their  posi- 
tion. Figure  282,  A,  represents  them  taken  in  the  center  of  the  mass. 

The  posterior  bodies  of  the  two  are  much  the  same,  except  that  the 
farther  caudad  they  are  found,  the  less  becomes  the  nerve-cell  portion. 
In  the  posterior  third  of  the  row  the  bodies  are  composed  chiefly  of 
chromaffine  cells. 

Besides  these  two  rows  of  paraganglia  there  may  be  found  another 
glandular  mass  without  any  duct  between  the  two  kidneys  near  their 
posterior  end.  Sections  of  this  material  (Fig.  282,  B}  show  that  it  is 
made  up  of  cortex  cells  which  appear  in  cords  and  masses  that  lie  in  a 
sinusoidal  plexus  of  blood  vessels.  The  cords  and  masses  are  usually 
two  cells  deep,  so  that  each  cell  has  a  proximal  end  in  contact  with  the 


THE  DUCTLESS   GLANDS 


315 


blood  supply,  from  which  it  is  separated  by  a  thin  layer  of  tissue.  Its 
distal  end  is  in  contact  with  the  distal  end  of  the  cell  opposite,  and 
although  there  is  no  lumen  visible,  there  is  a  tendency  for  the  collection 
of  secretion  granules  in  the  distal  cytoplasm.  In  another  fish,  the  conger, 
as  figured  by  Vincent,  a  lumen  does  occupy  this  region  and  probably 
acts  as  a  store  for  materials,  as  in  the  thyroid  gland. 

In  higher  forms  we  find  the  two  tissues,  just  described  in  the  skate, 
placed  in  varying  degrees  of  proximity  to  each  other.  In  the  teleost 
fishes  the  cortical  portion  is  large  and  separated  from  the  very  small 
amount  of  chromafrine  tissue.  In  the  Amphibia  the  two  are  in  closer 


FIG.  283.  —  Portion  of  a  section  through  the  adrenal  gland  of  a  fowl,  Callus  domesticus. 
ch.c.,  chromaffine  cells;  c.c.,  cortical  cells.     X  1000. 


relation  on  the  ventral  surface  of  the  kidney,  while  in  the  Sauropsida 
they  are  mingled  closely  in  a  single  gland  whose  position  much  resembles 
that  of  the  mammals. 

Figure  283  shows  a  section  of  the  adrenal  body  of  a  bird,  Callus  domes- 
ticus, in  which  one  can  recognize  the  larger  mass  of  cords  composed  of 
cortex  cells,  while  among  them  at  intervals  appear  smaller  groups  of 
chromafnne  cells.  The  cortex  cells  show  no  lumen  in  their  cords,  although 
they  are  arranged  so  that  every  mass  is  double.  Some  cells  are  even 
placed  in  a  cord,  so  that  they  do  not  have  access  to  the  blood  supply. 
The  body  of  such  a  cell  must  act  in  syncytial  unison  with  that  of  the 
cells  which  do  touch  the  blood  vessel. 

In  the  mammals  the  two  adrenal  tissues  are  placed  close  together,  the 


HISTOLOGY 


chromaffine  cells  forming  a  small  inner  medullary  portion  of  the  supra- 
renal body,  while  the  cortex  substance  forms,  as  its  name  implies,  an 

outer  cortical  part  (Fig.  284). 
The  cells  of  both  cortex  and 
medulla  form  irregular  cords, 
and  many  sympathetic  nerve 
cells  are  found  at  a  hilum  in 
contact  with  the  medulla. 
The  cords  of  cortex  cells  are 
extended  radially  from  the 
contact  with  the  medulla  to 
the  periphery  of  the  organ. 
The  blood  supply  is  carried 
between  the  cords  by  wide, 
irregular  capillaries  or  sinus- 
oids. A  more  internal  por- 
tion of  the  cortical  cords  is 
slightly  different  in  arrange- 
ment, and  forms  an  interme- 
diate zone.  The  medulla 
varies  in  amount  and  does 
not  extend  into  some  of  the 
smaller  lobes,  which  are  thus 
composed  of  cortex  alone. 
Another  gland  which  is 

FIG.  284.  —  Section  through  cortex  and  a  small  part  of    treated    of    here,    because    its 

function     is    not     Understood 


ch.  c. 


medulla  of  the  adrenal  gland  of  a  mole,     ch.c.,  chro- 
maffme  cells  of  medulla;  cor.,  cortex.     X  500. 


s    not 

and  because  it  has  no  duct, 
is  the  coccygeal  gland  of  man.  This  gland  may  be  dissected  out  as  a 
closely  associated  series  of  larger  and  smaller  masses  of  dense  yellowish 
tissue,  surrounding  and  adhering  to  the  branches  of  the  sacral  artery. 

A  section  of  one  of  these  masses  (Fig.  285)  shows  that  it  is  composed 
of  several  layers  of  the  specific  cells  of  the  gland,  adhering  to  the  very  thin 
walls  of  a  wide  blood  space  that  receives  blood  from  a  small  branch  of 
the  sacral  artery.  This  blood  space,  or  sinus,  empties  the  blood  through 
many  fine  vessels  which  pass  distally  through  the  gland-cell  layers  and 
collect  the  blood  to  deliver  it  into  a  neighboring  vein. 

The  cells,  as  shown  by  their  size,  more  compact  and  deeper  staining 
cytoplasm,  and  their  large  nuclei,  are  evidently  gland  cells  or  secreting 
cells.  They  are  not  arranged  so  as  to  have  any  neighboring  lumen 
and  therefore  must  return  their  elaborated  materials  to  the  blood.  Their 
portion  with  reference  to  the  blood  supply  is  accentuated  by  the  very  thin 
wall  of  the  blood  spaces  or  sinuses  as  well  as  by  the  fact  that  the  blood 


THE  DUCTLESS   GLANDS 


317 


must  pass  slowly  through  such  an  enlarged  vessel.    The  cells  are  thus 
in  an  advantageous  position  from  which  to  receive  food  materials  and 


FIG.  285.  — Section  through  a  lobule  of  the  coccygeal  gland  of  man. 
sec.c.,  secreting  cells;  bl.v.,  blood  vessel.     (After  WALKER.) 

oxygen  from  the  blood,  to  discharge  their  waste  matter  into  it,  and  also 
to  give  up  to  it  their  secretion  product,  which  must  be  of  some  use  to  the 
body  or  else  the  tissue  would  not  exist.  This  gland  is  found  in  the  body 
of  man  from  fcetal  life  to  death,  and  the  one  change  which  marks  its 
greater  age  is  a  larger  amount  of  connective  tissue  which  does  not  exist 
at  first  in  the  cell  masses. 

On  account  of  the  anatomical  position  of  the  gland  as  well  as  the 
structure  and  physiological 
reactions  of  its  specific  cells, 
it  has  been  supposed  to  be 
an  homologue  of  the  inter- 
renal  body  of  the  elasmo- 
branch  fishes.  This  idea 
must  remain  as  a  mere  spec- 
ulation for  the  present. 

Another  gland  of  interest 
is  the  carotid  gland,  which  is 
found  in  man  as  a  small 
mass  of  yellowish  red  tissue 
placed  at  the  bifurcation  of 
the  carotid  artery.  It  is  a 
little  larger  than  a  large  grain 
of  wheat,  and  might  be  com-  FlG  286  _  Section  of  the  carotid  goi  man.  secx., 

pared   closely  tO  the  COCCVg-         secreting  cells  surrounded  by  blood  channels.     (After 

eal  gland  as  to  its  structure.       SCHAPER.) 

It  has  much  the  same  kind  of  secreting  cells,  and  these  cells  border 

upon  a  blood  space  from  which  they  are  separated  by  the  very  thinnest 


31 8  HISTOLOGY 

of  walls,  a  single  layer  cf  endothelial  cells.  A  minor  difference  is  that 
the  cell  mass  is  divided  into  smaller  cords  and  plates  which  are  in  con- 
tact with  a  plexus  of  sinusoids  instead  of  with  a  few  larger  sinuses 
(Fig.  286). 

Technic.  — The  procedure  for  securing  specimens  of  these  tissues  is 
simple,  and  demands  but  one  detail,  the  use  of  some  salt  of  chromic 
acid  or  of  chromic  acid  itself  in  the  fixative  or  hardening  reagent.  This 
insures  the  peculiar  dark  brown  appearance  of  the  chromaffine  cells 
from  which  they  have  taken  their  name.  The  use  of  Flemming's  fluid 
or  of  Zenker's  fluid  is  thus  indicated  and  gives  the  best  results.  Much 
of  the  work  that  has  been  done  on  these  tissues  has  been  done  with 
Miiller's  fluid.  Such  work  has  been  unsatisfactory  in  all  details  except 
as  to  the  differentiation  of  the  chromaffine  cells. 


LITERATURE 

BERKLEY,  H.  J.     "The  Nerve  Elements  of  the  Pituitary  Body,"  Johns  Hopkins  Hospital 

Reports,  Vol.  IV,  1895,  p.  285. 
KURSTEINER,  W.     "Die  epithelial  koperchen  des  Menchen,"  Anat.  Heft,  Band  XI,  1898, 

S.  391. 
GOODALE,  J.  L.     "The  Endothelial  Phagocytes  of  the  Tonsilar  Ring,"  Journal  of  Medical 

Research,  Vol.  VII,  1902,  p.  394. 
KOHN,  A.     "Studien  iiber  die  Schilddriise,"  Arch.  f.  mik.  Anat.,  Band  XLVIII,  1897, 

s.  398. 

WELSH,  D.  A.  "Concerning  the  Parathyroid  Glands,"  Journal  of  Anatomy  and  Physi- 
ology, Vol.  XXXII,  1898,  pp.  292,  380. 

SCHAPER,  A.  "Zur  Histologie  der  Glandula  Carotica,"  Arch.  f.  mik.  Anat.,  Band  XL, 
1892,  S.  287. 

WALKER,  J.  W.  T.  "tiber  die  Menschliche  Steissdnise,"  Arch.  f.  mik.  Anat.,  Band 
LXIV,  1904,  S.  121. 

VINCENT,  SWALE.  "On  the  Comparative  Histology  of  the  Suprarenal  Glands,"  Internal. 
Monatschr.f.  Anat.  und  Physiologic,  Band  XV,  1898. 


CHAPTER    XVII 
TISSUES   OF   RESPIRATION 

The  respiratory  tissues  form  those  organs  by  means  of  which  an  ani- 
mal acquires  its  principal  supply  of  oxygen,  a  gas  that  is  absolutely 
needful  in  liberal  and  constant  supply  for  the  support  of  life.  Unlike 
some  food  materials,  oxygen  cannot  be  stored  for  any  length  of  time  in 
the  body,  and  therefore  the  organs  of  respiration  are  in  constant  use, 
even  during  sleep.  In  some  forms  sufficient  air  can  be  stored  in  these 
organs  to  last  for  a  short  time  while  the  animal  suspends  breathing 
temporarily.  Also,  in  other  forms,  an  extremely  slow  form  of  respira- 
tion takes  place  during  a  state  called  hibernation. 

The  oxygen  is  always  derived  from  the  free  supply  of  this  gas  in  the 
atmosphere.  When  water  is  breathed,  the  oxygen  is  also  obtained  from 
the  air  that  is  dissolved  in  the  water  to  which  it  has  access,  and  not  from 
the  oxygen  that  constitutes  a  part  of  the  water  chemically.  The  respira- 
tory tissues  also  serve  as  a  medium  through  which  carbon  dioxide,  a 
gas  resulting  from  the  use  of  the  oxygen  by  the  cells,  is  passed  out  of  the 
body.  The  exchange  of  these  two  gases  constitutes  respiration. 

The  specific  cell  of  respiration  is  a  thin  cell.  Besides  being  thin  in 
body,  a  feature  of  its  cytoplasm  is  its  clearness  and  non-staining  property, 
especially  of  that  part  of  it  which  is  most  directly  used  to  transmit  the 
gases.  This  probably  comes  from  the  absence  of  all  materials  or 
structures  that  might  impede  the  passage  of  the  oxygen  and  carbon 
dioxide.  It  is  always  placed  between  the  air  or  water  that  supplies  the 
oxygen  and  the  tissue  of  the  body  that  receives  it.  This  receiving  tissue 
is  usually  the  blood. 

The  exchange  of  gases  is  probably  not  due  to  any  specific  physiological 
action  of  these  cells,  but  rather  to  physical  and  chemical  laws  acting 
almost  unrestrainedly  through  the  body  of  the  cell  whose  function  seems 
to  be  one  of  self-elimination  in  the  processes  that  are  going  on.  This 
is  not  because  protoplasm  has  not  the  power  of  handling  gases  phy- 
siologically and  of  operating  with  them  against  the  activities  of  the 
ordinary  physical  and  chemical  laws  (read  next  chapter,  XVIII), 
but  rather  for  the  apparent  reason  that,  since  these  necessary  processes 
will  go  on  by  themselves,  it  would  be  a  loss  of  energy  to  the  organism  to 
do  it  physiologically. 

319 


320  HISTOLOGY 

The  respiratory  epithelium  is  one  of  the  most  generalized  tissues  found 
in  the  animal  body.  On  account  of  the  negative  nature  of  the  duties  to 
be  performed,  almost  any  epithelium  can  execute  them,  and  in  conse- 
quence, we  find  that  the  tissues  devoted  exclusively  to  respiration  may 
be  developed  on  a  great  variety  of  locations  of  the  integument  or  on  the 
inner  surface  regions  of  the  body.  Very  many  animals  have  no  specific 
surface  for  respiration.  Some  of  these,  too,  are  otherwise  highly  or- 
ganized, as,  for  instance,  the  earthworm,  which  utilizes  its  general  body 
surface  for  that  purpose.  This  use  of  the  whole  body  surface  might  be 
looked  upon  as  a  specialization  in  itself. 

The  lack  of  cytological  specialization  also  causes  as  great  a  diversity 
in  the  form  of  the  respiratory  organs  as  of  their  distribution.  This 
diversity  is  well  shown  in  the  worms,  for  instance,  where  some  of  them 
have  no  special  respiratory  organ,  while  others  have  them  in  a  variety 
of  forms. 

As  a  rule,  animals  breathing  water  use  an  evagination  of  some  sur- 
face for  a  respiratory  organ,  while  animals  that  derive  their  oxygen  from 
the  atmosphere  use  an  invaginated  surface  for  the  same  purpose.  In  the 
latter  case  the  organ  is  designed  to  protect  the  delicate  respiratory  cells 
from  the  effects  of  drying.  Cells,  strong  and  resistant  enough  to  stand 
drying,  would  not  easily  permit  the  gases  to  be  exchanged  and,  besides, 
the  moist  condition  is  the  most  favorable  for  the  process.  Water-breath- 
ing animals  may  live  in  the  atmosphere,  and  yet  carry  enough  water  in 
their  body  to  use  it  for  respiratory  purposes,  as  do  the  land  Crustacea 
and  some  fishes,  while  other  organisms  that  breathe  air  may  live  a  large 
part  of  their  lives  under  water,  and  at  the  same  time  carry  with  them 
the  air  that  they  breathe. 

Notwithstanding  the  negative  character  of  a  respiratory  epithelium, 
it  has,  in  many  cases,  retained  some  characteristics  of  the  epithelium 
from  which  it  has  been  derived.  These  characteristics  are  of  no  real 
use  to  it,  usually,  except  the  ciliated  condition  found  in  some  forms. 
Here  the  cilia,  which  only  appear  on  a  part  of  the  cells,  are  used  to  drive 
the  currents  of  water  over  the  respiratory  surfaces,  thus  performing  the 
act  of  breathing  which  in  other  organisms  is  performed  by  arrangements 
of  cavities,  valves,  and  muscles  that  belong  to  a  morphological  study  of 
the  subject.  Some  other  residual  characteristics  found  in  the  respiratory 
tissues  are  the  chitinous  covering  found  on  the  gills  of  the  Crustacea, 
worms,  etc.,  and  the  mucous  cells  and  pigmented  cells  that  can  be  seen 
on  the  gill  surfaces  in  other  forms. 

The  exchanges  of  gases  do  not  take  place  through  the  respiratory 
cells  alone.  When  the  blood  is  held  in  a  closed  circuit  of  vessels,  the 
walls  of  these  vessels  are  also  interposed  between  the  blood  and  the 
respiratory  medium.  Thus  the  gases  must  pass  through  two  layers  of 


LUNGS  321 

tissue,  the  walls  of  the  vessels  as  well  as  the  respiratory  cells.  That 
either  the  walls  of  the  respiratory  epithelium  or  the  walls  of  the  blood 
vessels  are  not  dispensed  with,  and  one  wall  used  to  separate  the  two  me- 
dia, is  possibly  due  to  a  number  of  reasons,  among  which  can  be  brought 
to  mind  a  lack  of  specialization  in  the  cooperation  of  the  two  tissues. 
This  may  not  have  occurred  because  of  the  lack  of  any  actual  need  of  it 
or  because  of  an  inherent  impossibility  of  an  ectodermal  epithelium 
becoming  the  wall  of  a  blood  channel,  or  of  a  connective  tissue  being 
situated  on  an  external  surface  of  the  body.  In  some  forms  it  is  very 
hard  or  even  impossible  to  detect  the  presence  of  this  blood-channel  wall. 

The  walls  of  the  blood  vessels,  where  they  are  in  contact  with  a  spe- 
cialized respiratory  epithelium,  are  as  thin  as  possible,  not  more  than  one 
cell  in  thickness.  This  single  layer  is  sometimes  so  thin  that  it  can  be 
seen  with  the  greatest  difficulty.  In  some  cases  it  is  clear  that  it  does 
not  exist,  thus  showing  an  exception. 

The  accessory  tissues  found  in  connection  with  the  specific  cells  of 
respiration  are  but  few  in  number.  A  small  amount  of  connective 
tissue  and  muscle  with  a  very  small  nerve  supply  are  all  that  are  directly 
concerned.  The  muscles,  cartilages,  and  other  tissues  of  the  breathing 
passage  and  gills  will  not  be  considered  here,  as  their  functions  are  but 
indirectly  related. 

Technic.  — These  tissues  may  be  treated  as  are  any  other  delicate 
epithelia.  When  well  hardened,  they  can  be  seen  in  section  and  dis- 
tinguished from  the  underlying  connective  tissues  by  their  hard  outlines 
and  compact  texture,  as  well  as  by  the  peculiar  transparent  appearance 
that  has  already  been  commented  upon.  Teasing  gives  no  important 
information,  and  is  best  put  aside  for  the  use  of  nitrate  of  silver  on  the 
fresh  tissue.  By  staining  the  cement  substances  in  this  way,  the  boun- 
daries of  the  cells  are  brought  out  and  their  relations  with  one  another 
clearly  demonstrated.  Lungs  should  be  gently  distended  as  in  life  at 
the  time  of  fixation,  in  order  that  the  elements  may  present  a  natural 
appearance.  Over  distention  is  more  harmful  than  the  reverse. 

LITERATURE 

See  the  general  text-books. 

AIR-BREATHING  RESPIRATORY   TISSUES 

The  respiratory  cells  of  the  salamander  are  found  on  the  inner  surface 
of  the  lung,  which  is  an  imagination  of  the  pharynx,  and  is  provided 
with  a  supply  of  fresh  air  by  the  breathing  of  the  animal.  These  cells  line 
the  entire  inner  surface  of  the  lung  and  rest  on  a  connective  tissue  with 
no  well-defined  basement  membrane.  Each  one  does  not  touch  this 


322 


HISTOLOGY 


connective-tissue  membrane  at  all  points,  but  only  with  a  comparatively 
small  surface  of  its  body,  while  the  rest  of  its  cytoplasm  broadens  out  into 
a  flange  that  reaches  away  from  the  pillar-like  supporting  mass  and  con- 
nects with  the  flanges  of  other  cells. 
Thus  an  arch  is  formed  between  the 
two  cells,  and,  as  the  cells  are  arranged 
in  groups  of  three,  all  of  whose  points 
of  contact  with  the  membrane  are  con- 
tiguous, there  is  formed  thereby  a 
network  of  channels  or  spaces  running 
everywhere  between  the  outer  cell- 
flanges  and  the  connective  tissue. 
These  channels  are  occupied  by  the 
network  of  capillaries  carrying  the 
blood  which  is  to  exchange  gases  with 
the  air-medium  in  the  lung. 

The  capillaries  have,  a  single-lay- 
ered wall  of  endothelial  cells,  and  this 
wall  is  closely  applied  to  the  flanges 
of  the  epithelial  cells  distad,  to  the 
connective-tissue  base  proximad  and 
to  the  attached  bodies  of  the  respira- 
tory cells  laterad  on  all  sides. 

In  the  section  (Fig.  287)  it  can  be 
seen  that  the  large,  round  nuclei  of 
the  respiratory  cells  are  nearly  always 
in  the  supporting  cell  mass  that  rests 
on   the    connective    tissue,   while   the 
elongated  (disk-shaped  in  surface  view) 
sections  of  the  nuclei  of  the  endothe- 
lial cells  that  form  the  walls  of  the 
blood  capillaries  are  to  be  found  at 
any  point  of  the  circumference  of  the 
FIG.  287. —Part  of  a  section  of  the  wall  of  section  of  the  capillary. 
rt"c^c«vrtt7cf:i,/:Se        This   arrangement    with    modified 
of  a  respiratory  ceil  (the  functional  part  detail   holds   for  the   vertebrate  lung 

t^lSZbSlSZZZi  to  e****-    Figure  288  shows  a  sur- 

face  view  of  the    respiratory  epithe- 

in  iii3.il. 

Another  form  of  air-breathing,  res- 
piratory tissue  is  found  among  certain  mollusks.  Mollusks  are 
ordinarily  water-breathers,  and  the  water-breathing  gill  or  cteni- 
dium  is  a  feature  of  much  morphological  importance.  In  a  group 


resting  on  the  connective  tissue;  ca.w., 
walls  of  the  blood  capillaries;  ca.nu.,  nu- 
clei  of  capillaries;  RC)  blood  cells.  X435- 


LUNGS 


323 


of  the  gasteropod  mollusks,  however,  the  ctenidium  is  not  present, 
and  another  organ,  an  invaginated  "lung"  is  formed  in  the  mantle 
cavity  to  use  air  as  a  respiratory 
medium. 

The  inner  surface  of  this  lung 
is  lined  with  the  respiratory  cells, 
which    must    be    extraordinarily 
efficient  if  their  structure  can  be 
taken  as  a  criterion  of  their  abil- 
ity as  a  medium  of  gas  exchange 
(Fig.  289).    They  are  so  flat  and 
thin  that  they  can  with  difficulty 
be  distinguished,  in  section,  from      ^ 
the   cells   they   rest   upon.     The        1 
nucleus  is  of  fair  size,  and  is  sev- 
eral times  the  general  thickness  of 
the  cell  body.    At  the  point  where    FIG- 
it  lies,  the  cytoplasm  is  thickened 
to  contain  it. 

In  the  lung  tissue  of  a  wood  snail,  Triodopsis  tridentata,  this  flat 
epithelium  covers  the  greater  part  of  the  cavity,  especially  such  parts 
as  are  provided  with  the  underlying  blood  channels.  The  cells  bear  no 
cilia,  but  in  some  parts  of  the  cavity  are  portions  of  the  epithelium  that 
are  composed  of  thicker  cells,  and  these  cells  are  provided  with  cilia. 
Such  organs  of  motion  are  necessary  in  a  cavity  that  is  constantly  in- 
vaded by  particles  of  foreign  substances  carried  with  the  air  supply. 
They  are  so  arranged  that  their  concerted  action  passes  the  foreign 


5.  —  A  surface  view  of  some  of  the  epi- 
thelium that  lines  the  mammalian  lung.  Ni- 
trate of  silver  preparation.  (From  Stohr's 
"  Histology,"  after  Lewis.) 


FlG.  289.  — Vertical  section  of  a  part  of  the  combined  body  wall  and  lung  wall  of  a  wood  snail, 
Triodopsis.  s.ep.,  external  shell  epithelium;  mus.f.,  two  layers  (longitudinal  and  circular) 
of  muscle  fibers;  bl.ca.,  layer  of  blood  capillaries,  one  of  which  contains  a  blood  corpuscle; 
c.w.,  single-layered  capillary  wall;  res.e.,  respiratory  epithelium.  It  clings  to  the  outer  sur- 
faces of  the  capillary  walls  in  nature  and  is  shown  artificially  separated  in  the  left  part  of 
the  figure.  X  730. 

particles  along  in  the  thin  covering  of   mucus    that  covers  them  and 
expels  the  whole  mass  from  the  pulmonary  opening. 

The  relations  of  the  respiratory  cells  to  the  blood  vessels  on  which 


324  HISTOLOGY 

they  rest  is  only  clearly  to  be  seen  when  a  happy  chance  shows  the  epithe- 
lium torn  partly  away  from  the  walls  of  these  vessels.  This  condition 
is  shown  in  Figure  289,  and  it  can  here  be  seen  that  the  respiratory  cells 
form  a  rather  even  layer,  and  do  not  have  any  portion  of  their  cytoplasm 
extending  down  between  the  blood  vessels  to  separate  them  and  form 
"tunnels"  for  them,  as  was  the  case  in  the  salamander.  Some  few  pro- 
cesses of  the  cytoplasm  do  dip  in  far  enough  to  secure  an  anchorage, 
but  this  is  of  small  extent  and  occurs  rather  seldom. 

The  blood  vessels  with  their  thin  walls  form  a  very  close  and  small- 
meshed  plexus.  On  this  account,  and  also  because  the  vessels  are  large 
as  compared  with  the  meshes,  all  sections  of  them  appear  to  be  trans- 
verse sections  or  slightly  oblique.  The  walls  of  the  vessels  are  formed  of 
a  single  layer  of  large  thin  cells,  whose  rather  widely  spaced  nuclei  appear 
but  seldom  in  the  section.  Coagulated  blood,  however,  containing  a  few 
typical  mollusk  blood  corpuscles,  fills  the  blood  channels.  Sections 
of  the  inter-vascular  spaces  or  islands  show  merely  a  few  connective- 
tissue  cells. 

The  blood  vessels  rest  on  a  longitudinal  and  a  transverse  layer  of 
muscle  fibers  that  lie  between  them  and  the  shell  epithelium  of  the  outer 
integument-  of  the  animal.  The  blood  must  derive  some  oxygen  through 
these  latter  thin  outer  layers. 

Respiratory  tissues  that  operate  without  the  intermediate  use  of 
blood.  — The  above  caption  is  not  strictly  true,  as  will  be  seen  from  the 
following  account,  but  it  will  serve  to  materialize  the  principle  involved. 
The  tissue  in  consideration  is  the  tracheal  respiratory  system  of  the 
insects.  This  structure  consists,  from  an  histological  point  of  view,  of 
an  imagination  of  the  same  tissues  that  were  evaginated  in  the  lobster 
and  other  Crustacea  to  form  gills. 

These  respiratory  tubes,  which  have  arisen  by  such  invagination  of 
the  surface  epithelium,  branch  and  rebranch  to  ultimately  form  minute 
ramifications.  It  is  the  source  of  much  controversy  as  to  whether  these 
ultimate  branches  anastomose  or  end  blindly  with  a  terminal  cell.  Be 
that  as  it  may,  this  invaginated  epithelium  is  carried  as  fine  tubes  to  all 
parts  of  the  body,  and  so  generally  distributed  that  all  tissues  can  be 
supplied  with  oxygen  directly  from  the  tracheoles.  Oxygen  is  thus 
distributed,  and  carbon  dioxide  collected  without  the  direct  intervention 
of  the  blood.  In  regions  of  great  blood  supply,  however,  the  plexus 
of  trachea  becomes  greatest,  and  by  means  of  such  plexuses  the  blood 
is  most  probably  charged  with  a  certain  amount  of  oxygen  for  distribu- 
tion. 

All  of  these  tracheae  are  composed  primarily  of  a  layer  of  epithelium, 
which  is  derived  from  and  continuous  with  the  hypodermis  of  the  body. 
The  epithelium  is  composed  of  flattened,  six-sided  cells  with  large 


GILLS 


325 


disk-shaped  nuclei  (Fig.  290).     They  bear  on  their  inner  surfaces  a  layer 

of  cuticle  which  they  have  elaborated.     As  we  saw  was  the  case  in  the 

water-breathing  crustacean  gill,  this  cu- 

ticle has  undergone  no  further  modifi- 

cation than  to  become  as  thin  as  possible. 

In  the  gill  of  the  Crustacea  the  pressure 

of  the  blood  was  positive  and  inflated 

the   evaginated    structure.     Here   the 

pressure  of  the  ccelomic  fluids  is  nega- 

tive, and  tends  to  collapse  the  tube. 

In    these    invaginated   tracheal   tubes, 

therefore,  the  cuticle  has  been  modified 

in  an  important  manner.     It  has  been 

thrown  into  thick,  circular  ridges,  the 

tanidia.    These  serve  to  keep  the  deli- 

cate tubes  open,  and  at  the  same  time 

do  not  make   the  walls  unnecessarily 

thick    and    heavy.     Similar    but    not 

homologous  formations  are   met  with 

in  the  tube  that  leads  to  the  large  lung    FIG.  290.  —  Part  of  an  oblique  section  of 

sacs  of  the  mammals  and  other  verte- 

bratCS.      In    this    case    there    is    also    a 

negative  pressure  during  expiration  and 

cartilaginous   rings    are    developed  to 

keep  it  from  collapsing.    The  conception  of  the  trachea  as  invaginations 

of  the  outer  surface  is  abundantly  borne  out  by  the  embryological  work 

on  this  structure.     They  can  be  seen  in  all  stages  of  invagination.    They 

do  not,  at  first,  contain  air,  this  appearing  at  an  early  stage. 

Technic.  —  One  must  be  somewhat  more  careful  in  fixing  these 
tissues  than  in  dealing  with  the  water-breathing  tissues.  When  once 
fixed  and  hardened  the  treatment  is  practically  the  same.  When  pos- 
sible, the  fixation  should  be  done  under  a  gentle  pressure.  The  harden- 
ing as  well  as  the  fixation  should  be  of  some  duration.  Often  it  is  of 
great  advantage  to  fix  by  inflating  the  organs  with  the  fumes  of  osmic 
acid  and,  after  this  has  had  plenty  of  time  to  act  (long  enough  to  "osma- 
tise"  the  respiratory  cells),  the  fixation  can  be  finished  by  the  'use  of  any 
other  fixative.  Silver  nitrate  used  on  the  fresh  tissues  may  be  made  to 
show  the  cell  outlines  very  beautifully. 


tube;  /.,  lateral  view  of  the  same;  /., 

^S6o°r  tenidia  of  the  cuticular  tube' 


LITERATURE 

MILLER,  W.  S.     " Das  Lungenlappchen  seine  Blut- und  Lymph-gefasse,"  Arch.  f.  Anat., 

1900,  S.  197. 
OPPEL,  A.     "Atmungs-Apparat,"  Erg.  d.  Anat.  und  Entwickl.,  1902,  Band  XII,  S.  134. 


326  HISTOLOGY 

BREMER,  J.  L.     "On  the  Lung  of  the  Opossum,"  Am.Journ.  ofAnat.,  1904,  Vol.  Ill,  p.  67. 

MILLER,  WM.  S.  "The  Lung  of  the  Salamander,  Necturus,"  Bull,  of  the  Univ.  of  Wis- 
consin, Nr.  33. 

PLATE,  LUD.  H.  "Studien  iiber  Opisthopneumone  Lungenschnecken,"  Zool.  Jahrb. 
Abt.fur  Anat.,  Band  IV. 

HOLMGREN,  E.  "Uber  das  respiratorische  Epithel  der  Tracheen  bei  Raupen,"  Festsk. 
Lilljeborg.,  Upsala,  1896,  pp.  79-96. 


WATER-BREATHING  RESPIRATORY   TISSUES,   GILLS 

The  water-breathing  forms  of  respiratory  tissues  are  the  most  primi- 
tive; at  the  same  time  their  distribution  is  most  diverse  and  their  varia- 
tion is  greatest. 

As  an  example  of  a  simple  water-breathing,  respiratory  organ  we 
shall  take  the  primary  gill  filaments  of  the  embryo  of  Acanthias  vulgaris 

(Fig.  291).  In  the  em- 
bryo of  most  selachians 
the  animal  secures  its 
oxygen  and  carbon  dioxide 
exchange,  with  the  embry- 
onic fluids  in  which  it  lies, 
by  means  of  a  series  of 
long  filaments  that  grow 

FIG.  291.— Transverse  section  of  an  embryonic  respira-      out  frOm    the    sides  of   the 
tory  filament  of  Acanthias.     X  400.  i      •         i  -11 

neck  in  the  gill  region. 

Each  filament  consists  of  a  long,  single  capillary  loop  embedded  in  a 
very  small  amount  of  connective  tissue,  and  surrounded  by  an  evaginated 
layer  of  the  body  epithelium.  The  blood  passes  down  one  side  of  the 
filament  and  returns  on  the  other.  The  afferent  and  efferent  capilla- 
ries that  are  seen  in  the  section  of  the  filament  are  lined  with  a  single 
layer  of  endothelial  cells  and  contain  fully  developed,  red,  nucleated 
corpuscles. 

In  the  somewhat  oval  section  of  such  a  filament  we  find  the  respiratory 
cells  to  be  a  single  layer  of  cells,  a  little  too  flat  to  be  called  cubical,  and 
differing  but  little  from  the  cells  on  the  surface  of  the  body  from  which 
they  were  derived,  except  that  these  latter  are  already  stratified  in  a 
four-centimeter  embryo  into  two  layers  or  more. 

The  small  amount  of  connective  tissue  that  is  seen,  forms,  for  the 
most  part,  a  septum  separating  the  two  vessels  from  one  another.  A 
few  of  these  cells  are  to  be  found  between  the  capillary  and  the  respiratory 
epithelium  but  they  are  very  much  flattened.  Undoubtedly  parts  of 
the  cytoplasm  of  these  cells  separate  the  vessels  from  the  epithelium  at 
every  point.  The  triple  wall  of  epithelium,  connective  tissue,  and  en- 
dothelium  is  a  fairly  efficient  organ  for  the  transmission  of  gases  when 


GILLS 


327 


the  great  length  of  the  single  filament  is  considered,  and  the  fact  that  there 
is  an  exposure  of  the  blood  to  the  oxygen  supply  along  its  entire  length. 
A  characteristic  form  of  respiratory  membrane  is  to  be  seen  in  the  gill 
of  the  lobster.  A  long  axis  with  nervous,  muscular,  and  other  structures 
bears  a  stream  of  blood  out  to  its  end  and  sends  it  off  into  a  series  of 
filaments.  In  cross  section,  such  a  filament  is  oval  (Fig.  292),  and  its 


FIG.  292.  —  Part  of  a  transverse  section  of  a  respiratory  filament  of  the  lobster's  gill,  bl.v., 
afferent  and  efferent  blood  vessels;  bl.ca.,  capillaries;  hy.,  hypodermal  (epidermal)  cells 
which  line  the  body  surface  and  produce  the  cuticle;  cu.,  cuticle;  ep.lam.,  thin  layer  of  epi- 
dermal cytoplasm  lying  between  a  respiratory  capillary  and  the  cuticle;  conn.t.,  connective- 
tissue  nuclei ;  x,  unknown  bodies  near  wall  of  one  blood  vessel.  Arrows  show  how  blood 
passes  from  artery  (afferent  blood  vessel)  to  vein  (efferent  vessel).  X  725.  v 

outer  edge  is  formed  by  a  layer  of  covering  cells  in  a  loose  mass  of  con- 
nective tissue  containing  the  blood  vessels  of  the  filament.  There  is 
no  basement  membrane  to  separate  the  hypodermis  from  the  connective 
tissue. 

All  blood  spaces  in  the  filaments  appear  to  be  channels  lying  among 
the  Leidig's  connective-tissue  cells,  and  they  are  of  two  groups.  First, 
there  are  the  afferent  artery  and  an  efferent  vein  that  together  form  a 


328  HISTOLOGY 

loop  to  carry  the  blood  into  and  out  of  the  filament  as  was  done  in  the 
dogfish  embryo.  Our  figure  shows  one  half  of  a  section  through  a  fila- 
ment, and  one  of  the  large  blood  vessels,  the  artery. 

But  this  vascular  loop  does  not  form  a  direct  pathway.  The  second 
set  of  vessels  are  a  set  of  fine  capillaries  that  serve  to  carry  the  blood  from 
the  artery  to  the  vein  along  their  entire  course.  They  leave  the  artery 
on  its  outer  edge  and  extend  around,  as  a  plexus  lying  close  to  the  surface, 
to  empty  into  the  outer  side  of  the  vein.  They  thus  keep  a  large  amount 
of  the  blood  close  to  the  surface,  and  in  a  suitable  position  for  gas  ex- 
change or  respiration  to  go  on. 

These  capillaries  appear  to  have  no  walls  of  their  own,  but  to  pass 
between  the  connective-tissue  cells,  between  these  and  the  hypodermis 
cells,  or  even  between  the  hypodermis  cells  and  the  cuticle.  They  never 
quite  reach  this  cuticle,  however,  as  a  small  plate  of  cytoplasm  belonging 
to  the  hypodermis  cells  always  keeps  them  from  directly  touching  it. 
This  is  well  shown  at  ep.  lam.  in  Figure  292. 

The  connective-tissue  cells  that  form  the  central  core  of  this  filament 
are  more  characteristic  than  any  others  in  the  lobster's  body.  They  do 
not  show  the  periphery  that  the  Leidig  type  of  cell  does,  but  have  loosely 
branched  protoplasmic  processes.  A  very  peculiar  set  of  round  objects 
which  somewhat  resemble  nuclei  are  found  on  the  inner  side  of  the  artery 
and  are  shown  at  x  in  Figure  292. 

All  the  blood  vessels,  even  the  small  capillaries,  are  lined  with  a  very 
thin  cuticular  substance  which  gives  them  a  clear  and  unmistakable 
outline  that  is  well  shown  in  the  drawing.  It  should  be  remembered 
again  that  the  surface  of  this  gill  consists  of  the  same  elements  that  the 
crustacean  or  insect  body  does,  of  an  epithelium  or  hypodermis  which 
secretes  a  cuticle,  here  modified  by  thinning  for  a  special  purpose.  The 
organ  is  evaginated  because  it  it  to  be  used  in  water.  If  it  were  to  be 
used  in  air,  it  would  be  invaginated  as  it  is  in  the  insects. 

Note  the  thin  layer  of  cytoplasm  lying  between  the  blood  capillary  and 
the  cuticle  at  ep.  lam.  in  Figure  292.  This  is  not  to  keep  the  blood  from 
touching  the  cuticle,  but  to  provide  the  cuticle  with  a  portion  of  cytoplasm 
which  is  the  only  agent  which  can  make  it  and  renew  it  when  necessary. 

Some  of  the  gills  found  on  worms  are  remarkable  structures,  and 
bear  interesting  histological  relationships.  The  gill  filaments  of  the 
worm,  Amphitrite  ornata,  are  good  examples  and  a  transverse  section 
of  one  of  these  long,  extensible  filaments  will  show  the  desired  features 
(Fig.  293).  These  filaments  are  used  for  other  purposes  than  respira- 
tion, and  it  is  not  known  whether  the  animal  would  perish  at  once,  for 
lack  of  oxygen,  without  them.  It  probably  would  not,  but  would  have 
.time  to  regenerate  them. 

As  can  be  seen,  the  filaments  consist  of  a  long  core  of  connective 


GILLS 


329 


tissue  containing  two  blood  vessels  that  lie  near  opposite  sides  of  the 
filaments.  This  core  is  composed  of  a  very  delicate  connective  tissue,  and 
two  bands  of  longitudinal  muscle  fibers  lie  on  the  edges  farthest  from 
the  blood  vessels. 

The  whole  structure  is  covered  with  a  tall,  heavy,  columnar  epithe- 
lium whose  cells  show  but  poor  lateral  boundaries  owing  to  the  intimate 
way  in  which  they  are  cemented  together.  A  row  of  ciliated  cells  ex- 
tends for  the  length  of  the  filament  on  one  side. 


.ctt. 


FIG.  293. — Transverse  section  of  a  respiratory  filament  (tentacle)  of  an  annelid  worm,  Am- 
phitrite  ornata.  bl.v.,  blood  vessels,  smaller  afferent  vessel  or  artery,  larger  efferent  vessel 
or  vein;  bl.c.,  capillaries  which  conduct  the  blood  from  afferent  vessel  to  efferent  vessel 
through  the  respiratory  epithelium;  mus.f.,one  of  the  two  bands  of  longitudinal  muscle 
fibers;  cil.,  ciliated  cells  on  one  edge  of  filament.  X  400. 

One  of  the  blood  vessels  is  large  and  the  other  smaller.  This  latter 
is  probably  an  artery  through  which  the  blood  runs  faster,  while  it  runs 
slower  through  the  wider  vein.  The  blood  does  not  pass  through  the 
artery  and  vein  as  a  simple  loop.  Instead  it  passes  out  of  the  artery, 
on  its  distal  edge,  into  a  great  number  of  fine  capillaries  which  pass 
both  ways,  through  the  epithelium,  around  to  the  distal  edge  of  the  vein, 
which  they  enter.  The  blood  is  thus  brought  into  extensive  and  in- 
timate contact  with  the  outer  surface,  and  is  thus  aerated  and  enabled 


330 


HISTOLOGY 


to  throw  off  such  impurities  as  it  can  in  contact  with  the  air  (dissolved 
in  water).  No  covering  could  be  detected  between  the  blood  stream 
and  the  surrounding  epithelial  cells  in  which  the  vessel  lay  embedded. 
This  form  of  tissue  is  much  like  that  of 
the  lobster. 

The  mollusks  that  breathe  water  have 
a  varied  assortment  of  evaginated  fila- 
ments, plates,  etc.,  which  are  in  some 
cases  exceedingly  complicated.  We  shall 
study  the  conditions  first  as  shown  in  the 
gill  of  the  prosobranch  gasteropod,  Syco- 
typus  (Fig.  294).  This  gill  is  a  double 
series  of  plate-  or  leaf-like  evaginations, 
between  whose  double  walls  the  blood 
slowly  flows  in  very  irregular  capillary- 
like  sinuses.  These  vessels  are  separated 
from  the  proximal  edge  of  the  epithelium 
by  an  abundance  of  loose  connective  tis- 
sue. 

The  epithelium  is  columnar  and  dense, 
and  is  as  unspecialized  a  form  of  respir- 
atory membrane  as  we  have  encoun- 
tered. It  is  ciliated  in  many  places  and 
contains  many  mucous  cells.  The  base- 
ment membrane  on  which  the  cells  lie  is  well  defined,  but  very  thin  in 
most  places.  On  the  two  sides  of  the  edge  of  each  lamella  it  is  thickened 
into  two  rods  which  are  almost  crescent-shaped  in  section.  These  are  to 
be  considered  as  skeletal  structures  used  to  stiffen  the  gill  plate  (Fig. 

295)- 

The  essential  histological  points  of  this  respiratory  tissue  may  be  seen, 
accompanied  by  far  more  complicated  anatomy,  in  the  ctenidia  of  other 
mollusks.  The  squid  and  various  lamellibranchs  afford  structures 
that  will  repay  study  by  the  exhibition  of  marvelous  adaptations.  Most 
of  these  other  forms  show  a  more  specialized  epithelium ;  one  whose  cells 
have  given  up  other  functions,  and  become  as  thin  in  body  and  as  clear 
in  cytoplasm  as  possible  to  permit  of  the  ready  passage  of  gases  through 
their  bodies. 

The  respiratory  tissues  of  the  fishes  are  found  on  evaginations  and 
growths  from  the  branchial  arches.  These  take  the  form  of  a  series  of 
lamellae  which  have  arisen  from  the  epithelium  and  its  supporting  con- 
nective tissue. 

In  the  gill  of  the  goldfish  the  general  surface  of  the  gill  plate  is  cov- 
ered by  a  stratified  epithelium.  The  basal  layer  of  this  epithelium  is 


FIG.  294. —  Central  part  and  epithelium 
on  one  side  of  a  gill  plate  of  Sycotypus. 
bl.sp.,  blood  space  containing  a  thin 
coagulum  and  a  few  blood  cells; 
res.ep.,  respiratory  epithelium,  x 


GILLS 


331 


thrown  into  ridges  which  by  continued  evagination  rise  uncovered  be- 
yond the  general  surface  of  the  stratified  epithelium.  These  evagina- 
tions  carry  with  them  between  their  two  walls  a  vascular  plexus  and  a 
connective-tissue  support  (Fig.  296). 

The  blood  vessels  have  a  true  capillary  structure  with  definite  en- 
dothelial  walls  of  their  own  so  that  the  contained  blood  is  never  in  actual 
contact  with  the  respiratory  epithelium.  These  walls  are  thinnest  where 


FIG.  295.  —  Transverse  section  of  the  edge  of  a  respiratory  plate  of  Sycotypus.  rd.,  one  of  the 
two  chitinous  rods  which  support  the  edge.  The  blood  space  shows  blood  cells  and  the 
epithelium  is  furnished  with  cilia.  X  700. 


they  are  next  to  the  respiratory  cells  to  permit  of  the  freest  gas  exchange. 
They  are  decidedly  thicker  and  heavier  where  they  lie  in  the  meshes 
of  the  plexus  or  between  the  capillaries  as  they  appear  in  the  figure. 
This  is  probably  to  afford  support  for  the  lamella.  The  epithelium  as 
a  whole  must  be  carefully  studied.  Where  it  remains  on  the  surface 
of  the  gill  bar  and  between  the  lamellae  it  is  typically  stratified,  the  basal 
layer  proliferating  freely  and  the  proliferated  layers  lying  in  a  mass  that 
reaches  halfway  up  between  the  lamellae.  The  cells  of  the  uppermost 
or  superficial  layer  of  this  mass  are  enlarged,  and  dense  with  the  nucleus 


332 


HISTOLOGY 


situated  proximally  in  their  bases  (Fig.  296,  ep.c.).    One  or  two  of  such 
cells  will  be  found  between  most  lamellae.     Any  particular  function  which 

they  may  possess  is  unknown. 

Those  cells  of  the  basal  layer 
which  are  reflected  over  the  la- 
mellae become  the  true  respiratory 
cells  (Fig.  296).  In  this  position 
they  have  relinquished  the  work 
of  desquamation,  and  consequently 
remain  a  simple  epithelium. 
Their  renewal,  when  injured  or 
worn  out,  is  secured  by  mitotic 
cell-division  in  the  fundus  and  on 
the  lower  sides  of  the  lamellae,  and 
the  moving  of  the  whole  layer  up 
to  fill  the  gap.  A  mitotic  figure 
which  is  probably  fulfilling  this 
duty  is  shown  on  the  middle  side 
of  the  lamella  in  the  figure. 

In  structure  the  respiratory 
cells  are  decidedly  specialized. 
They  do  not  become  so  flat  and 
thin  as  many  other  cells  do  under 
the  circumstances,  but  they  broad- 


FIG.  296.  —  Part  of  a  section  through  the  gill 
filament  of  a  goldfish,  res.c.,  respiratory  cells; 
bl.c.,  blood  cells  in  the  many  sections  of  cap- 
illaries; ep.c.,  unmodified  outer  cells  of  the 
stratified  epithelium  from  which  the  gill  is 
developed  (possibly  has  some  obscure  glandu- 
lar function).  X  1300. 


en  and  become  lighter,  and  the 
cytoplasm  becomes  far  less  dense 
than  it  is  in  the  lower  parts. 

Technic.  — The  gill  tissues  are 
very  easy  to  cut  in  paraffin  on  ac- 
count of  the  delicacy  of  the  vari- 
ous structures  which  have  to  be  so  because  of  the  necessity  of  allowing 
the  oxygen  to  pass  through.  For  the  same  reason  the  organs  are  quite 
hard  to  fix  without  shrinkage  and  distortion.  Flemming's  fluid,  Zen- 
ker's  fluid,  and  chrom-aceto-formaldehyde  gave  good  results,  especially 
when  allowed  to  act  for  a  long  while. 


LITERATURE 

REISS,  A.     "Der  Bau  der  Kiemenblatter  bei  den  Knockenfischen,"  Troschler's  Arch.fiir 

Naturges.,  47  Jahrgang. 
OSBORN,  H.  L.     "On  the  Gill  in  Some  Forms  of  Prosobranchiate  Mollusks,"  Stud,  from 

the  Biol.  Lab.,  Johns  Hopkins  University,  Vol.  Ill,  1884. 
MOROFF,  TH.     "Uber  die  Entwicklung  der  Kiemen  bei  Knockenfischen,"  Arch.  f.  mik. 

Anat.,  Vol.  LX,  1902. 


CHAPTER    XVIII 
THE  GAS-SECRETING  TISSUES   OF  ANIMALS 

AMONG  the  many  substances  that  cells  can  produce  by  the  process 
of  secretion  is  free  gas.  Of  course  we  have  seen  in  the  preceding  part  that 
very  many  tissues  can  allow  gas  to  be  transmitted  through  their  substance 
under  physical  and  chemical  impulse  (pressure  and  chemical  affinity). 
But  the  cells  to  be  discussed  in  this  part  are  able  to  take  the  gas  materials 
from  the  blood  and  to  secrete  and  discharge  them  into  a  chamber  that  is 
under  a  pressure  greater  than  that  of  the  surrounding  medium  in  which 
the  animal  is  placed.  As  several  gases  are  so  handled  as  a  mixture, 
there  are  many  unknown  chemical  changes  involved.  The  gas  appears 
as  tiny  solid  granules  in  the  cytoplasm  of  the  cell  and  swells  into  a  fluid 
droplet,  and  then  into  the  gaseous  state,  when  it  is  discharged  from  the 
cell. 

We  shall  examine  two  of  the  very  few  forms  of  animals  in  which  this 
occurs,  a  teleost  fish  whose  swim-bladder  is  filled  with  a  mixture  of  gases 
and  a  siphonophore  medusa,  one  of  whose  zooids  is  developed  into  a 
hollow  float  that  is  also  filled  with  about  the  same  mixture  of  gases 
that  we  found  in  the  swim-bladder  of  the  fish.  This  mixture  is  CO2  in 
2-6 /fc;  Oxygen,  12-18^;  Nitrogen  79-80^. 

Gas  secretion  in  the  siphonophore  medusa,  Physalia,  and  others.  — 
The  float  member,  of  the  collection  of  individuals  that  a  Physalia  repre- 
sents, is  developed  by  an  invagination,  into  a  large  hollow,  double- 
walled  sac  or  bladder  with  a  pore  at  one  point  that  controls  the  exit, 
and  the  consequent  pressure  of  the  gases  by  a  sphincter  muscle.  When 
gas  is  lost,  by  letting  it  out  or  when  the  supply  is  decreased  by  the  growth 
and  enlargement  of  the  float  or  by  the  loss  of  gas  by  osmosis  through 
the  walls,  a  new  supply  is  provided  by  a  portion  of  the  epithelium  that 
is  situated  on  the  inner  membrane  near  the  base  of  the  float  (Fig.  297). 

This  epithelium,  which  lines  the  membrane  and  faces  the  hollow  of 
the  float,  consists  of  a  single  layer  of  long  cells  with  a  swollen  distal  por- 
tion that  narrows  down  into  a  rod-like  base  near  the  basement  membrane, 
where  it  branches  into  several  root-like  processes  that  are  implanted  in 
the  jelly  tissue  of  the  mesoglcea.  The  nucleus  is  of  large  size  and  placed 

333 


334 


HISTOLOGY 


chr. 


where  the  basal  portion  of  the  cell  begins  to  widen,  about  one  third  of 
the  distance  from  the  base.    The  secretion  first  appears  as  a  group  of 

small  granules  immediately  distad  of 
the  nucleus,  and  these  granules  move 
toward  the  distal  end  of  the  cell, 
where  they  swell  and  become  filled 
with  the  gas.  The  large  gas  bubbles 
rupture  the  cell-wall  and  break  into 
the  gas  chamber  to  supply  it.  The 
whole  membrane  with  its  epithelium  is 
thrown  into  a  series  of  parallel  folds 
of  moderate  depth.  These  folds  be- 
come of  lesser  depth  on  either  side  of 
a  central  area. 

A  short  account  of  the  gas  cells  of 
another  siphonophore,  Physophora  hy- 
drostatica,  which  occur  in  a  more 
highly  specialized  organ,  should  be 
considered  here. 

The  gas  cells  in  this  form  are 
found  on  a  membrane  homologous  to 
t^iat  wn*cn  bears  them  in  Physalia. 
But  all  of  the  epithelial  cells  on  this 
membrane  are  not  developed  into  the 
gas  cells.  Most  of  them  are  a  simple 
cell  representing  the  secondary  ecto- 
derm (from  which  the  gas  cells  also 
arise)  in  its  simplest  form.  Only,  in- 
stead of  both  sorts  of  cells  forming  a 

FIG.  297.  —  Two  gas-secreting  cells  from  ,  .  ,.„  ,         .. 

the  gas  epithelium  of  the  siphonophore    single  row,  the  undifferentiatcd  cells 
medusa,  Physalia.    v.,  small  vacuoies;    form  a  thick,  many-layered  mass  in 

chr.v.,    the     chromatic    vacuoies:     b.,          i  •   i    -i  11  t         j  i 

bundles  of  mesodermal  tissue  belonging     wmch  the  gas  cells  are  Placed  Sparingly 

b.,  smaller  mesoder-  and  always  away  from  contact  with 
both  basement  membrane  and  distal 
surface. 

These  cells  secrete  the  gas  in  much 
the  same  way  that  it  is  done  in  Physalia  except  that  the  vacuoies  of 
gas  must  force  their  way  to  the  surface  and  break  out  into  the  gas 
chamber. 

An  even  more  highly  specialized  form  of  gas  cell  is  found  in  a  third 
form  of  siphonophore,  Rhizophyza  filiformis,  which  occurs  in  the  Medi- 
terranean Sea  (Fig.  298).  This  large  gas  cell  has  a  huge  nucleus  of 
peculiar  texture,  which  is  shaped  like  a  kidney.  On  the  hollowed  side 


to  gas  epithelium; 

mal  bundles  belong  to  the  corresponding 

endodermal    epithelium,   the    bases    of 

whose  cells  are  indicated  by  lines.     X 

1000. 


GAS-SECRETING    TISSUES 


335 


of  this  nucleus  is  a  sphere  of  darker  staining  cytoplasm  that  appears 
much  like  the  centrosphere  of  some  spermatogonia.  Around  this  sphere 
is  a  zone  of  lighter 
staining  substance 
that  is  yet  darker 
than  the  cytoplasm, 
and  whose  ends  are 
expanded  on  the  side 
Jthes,  frora  the  nu- 
cleus  into  two  wing- 
like  processes  that 
reach  almost  to  the 
cell-wall.  The  secre- 
tion appears  in  the 
form  of  fine  granules 
that  swell  and  finally 
are  transformed  into 
the  gas  near  the  pe- 
riphery of  the  cell. 

Gas  secretion  in 
the  teleost  fish,  Gadus 
morhua,  and  others. 
— Many  of  the  teleost 
fishes  possess  a  swim-bladder  that  serves  to  reduce  their  specific  grav- 
ity by  secreting  and  containing  a  gas  mixture.  The  gas  is  secreted  by 
the  epithelial  lining  of  the  organ. 

As  the  swim-bladder  is  formed  by  an  embryonic  invagination  of  the 
intestinal  tract,  this  gas  epithelium  is  genetically  related  to  the  respiratory 
cells  of  the  vertebrate  lung,  whether  the  swim-bladder  and  the  lung  are 
the  same  or  different  invaginations,  phylogenetically,  of  this  region  or  not. 

But  while  the  respiratory  cells  passively  allowed  various  gases  to 
diffuse  themselves  through  their  cytoplasm,  this  epithelium  of  the  swim- 
bladder,  as  has  been  said,  secretes  it  into  a  chamber  that  is  under  a 
mechanical  pressure,  and  perhaps  a  chemical  condition  of  resistance 
as  well. 

The  epithelium  sometimes  covers  the  entire  inner  surface  of  ftie  swim- 
bladder,  while  in  our  subject,  the  cod,  only  that  portion  on  a  limited  area 
of  this  lining  epithelium  is  so  used.  This  smaller  part,  however,  is 
specialized  into  a  condition  of  greater  efficiency  by,  first,  an  amplification 
of  its  surface  through  numerous  tubular  and  folding  invaginations  and, 
secondly,  by  an  increase  in  the  size  and  thickness  of  the  secreting  cells 
themselves  (Fig.  299).  This  is  accompanied  by  an  increased  peripheral 
blood  supply  that  is  pushed  up  in  capillary  loops  into  the  regions  between 


FIG.  298.  —  Part  of  a  gas-secreting  cell  from  the  siphonophore 
medusa,  Rhizophyza  filiformis.  nu.,  nucleus  showing  a  dark 
and  crowded  chromatin  pattern;  x,  unknown  centrosphere- 
like  body.  (After  K.  C.  SCHNEIDER.) 


336 


HISTOLOGY 


the  invaginations.  A  peculiarity  of  this  blood  supply  is  the  way  that  the 
blood  stream  is  divided.  The  dendritic  method  of  division  found  in  the 

circulatory  supply  of  most 
organs  and  tissues,  where 
the  artery  divides  into 
branches  and  subdivides  un- 
til the  stream  is  running  in 
capillaries,  is  replaced  by  a 
great  mass  of  capillaries  that 
arise  together  on  a  few  large 
vessels  that  are  found  on  the 
inner  surface  of  the  bladder 
and  run  in  a  mass  with  par- 
allel courses  to  the  secreting 
epithelium.  Here  they  enter, 
and  having  entered,  they  be- 
gin to  separate  from  one 
another.  Toward  the  outer 
surface  of  the  mucosa,  as  we 
shall  call  the  layer  of  invagi- 
nated  epithelium  together 
with  the  connective  and  other 
tissues  that  are  involved  with 

FIG.  299  —Part  of  the  distal  region  of  the  gas-secreting    jt    fa  capiHaries  anastomose 
gland  of  the  cod,  Gadus.     bl  c. ,  blood  capillaries  with  11  i  •    i 

endothelial  walls  on  which  the  proximal  surfaces  of    and    become    Slightly    larger. 

the  gas  cells  rest;  /.,  parts  of  two  lumina  on  which  They  may  be  designated  sinu- 

the  distal  ends  of  the  gas  cells  border,     x  1000.  »i      i  o  T  i 

soids  here.     So  complicated 

have  the  histological  relations  become  in  this  mucosa  that  the  epithelial 
nature  of  the  secreting  cells  is  doubtful  without  careful  study.  Many  inner 
cells  are  apparently  devoid  of  a  disto-proximal  differentiation,  owing  to 
the  fact  that  the  lumen  of  the  gland  is  closed  by  the  absence  of  any 
secretion  at  the  point  where  they  are.  In  places  where  the  comparatively 
scarce  lumina  of  secreting  acini  push  their  way  down  into  the  mass,  the 
relations  of  the  cells  are  clearly  seen,  and  in  some  cases  they  appear  typi- 
cally columnar,  resting  on  the  blood  channels  from  which  they  draw  their 
materials  with  the  even  surface  of  their  ends  bounding  the  round,  open 
lumen.  Such  a  lumen  was  probably  full-  of  gas  at  the  time  of  fixa- 
tion. 

The  epithelium  directly  on  the  primary  surface  of  the  gas  gland  is 
slightly  different  from  the  cells  bordering  on  the  secondary  lumina,  and 
probably  have  in  addition  to  their  duty  of  gas  production  the  work  of 
secreting  the  peculiar  layer  that  lines  the  entire  inner  surface  of  the  swim- 
bladder.  The  gas  bubbles  slowly  force  their  .way  through  this  layer, 


GAS-SECRETING    TISSUES 


337 


which  parts  before  the  pressure  and  closes  again  when  the  bubble  has 
passed. 

The  cytology  of  the  typical  gas  cells  is  peculiar.  As  this  is  more 
easily  seen  in  those  of  some  other  fish  than  the  cod,  we  shall  examine 
them  in  the  swim-bladder  of  the 
golden  paradise  fish,  or  so-called  Jap- 
anese goldfish  (Fig.  300).  This  tis- 
sue has  been  described  by  Reis  and 
Nusbaum. 

In  this  form  the  differentiation  of 
a  distinct  gland  is  but  partial,  the 
thick  heavy  gas  cells  lining  the  entire 
interior  of  the  swim-bladder,  and 
being  invaginated  to  some  degree  on 
an  area  of  the  ventral  surface  only. 
Here  the  few  tubular  invaginations 
extend  down  into  the  connective  tis- 
sue lying  between  the  epithelium  and 
the  wall  of  the  swim-bladder. 

Cells  taken  from  any  part  of  the 
gas  epithelium  will  answer.  Such 
a  cell  is  cylindrical  with  its  base  rest- 
ing on  a  blood  channel  and  its  distal 

end  touching  the  lumen  of  the  bladder  or  one  of  its  branches  in  the  lu- 
mina.  Its  base  is  somewhat  thickened  and  stains  deeper  owing  to 
materials  placed  here  that  do  not  occur  elsewhere.  These  materials, 
which  at  best  can  be  only  identified  as  fine  granules  amid  a  network  of 
irregular  fibrils,  are  either  some  particular  organ  of  the  cytoplasm  that 
is  used  to  collect  the  gas-forming  materials  from  the  blood,  or  they  repre- 
sent those  collected  materials  themselves.  A  combination  of  both  con- 
ditions is  the  best  explanation  of  this  appearance. 

These  materials  must  certainly  be  passed  forward  through  the  cyto- 
plasm of  the  cell  as  fluids  or  as  very  fine  granules.  This  can  safely  be 
assumed,  yet  no  trace  of  their  presence  is  visible  until  we  have  gone  distad 
of  the  nucleus  in  our  search,  and  come  to  the  outer  third  of  the  cell.  Here 
the  materials  for  the  making  of  the  gas  are  once  more  to  be  seen,  collected 
as  granules  in  an  important  organ  of  the  cell,  a  series  of  fine  cytoplasmic 
channels  that  gather  at  their  center  into  a  central  cleft  of  irregular  out- 
line. The  lumen  of  this  channel  system  is  not  free  but  filled  with  another 
and  more  fluid  form  of  cytoplasm.  The  cytoplasm  of  this  region  elabo- 
rates the  materials  into  the  gas  or  into  granules  of  substances  that  readily 
combine  to  form  the  gas,  and  these  granules  are  passed  down  the  channels 
to  the  cleft.  In  this  space  they  are  converted  into  the  gas  mixture  which 


FIG.  300.  —  Five  gas-secreting  cells  from 
the  gas  gland  in  the  swim-bladder  of  the 
paradise  fish,  Macropodus  viridi-auratus. 
b.,  thickened  distal  border  of  the  cells 
on  the  lumen;  vac.,  gas-vacuoles;  lr., 
trophospongia;  bl.ca.,  capillary.  (After 
REIS  and  NUSBAUM.) 


338  HISTOLOGY 

appears  as  a  number  of  tiny  bubbles  that  unite  to  form  larger  vesicles. 
These  latter  work  to  the  surface  of  the  gland  or  inner  surface  of  the  swim- 
bladder  and  are  discharged  into  it. 

When  the  gas  is  formed,  there  is  a  residual  material  that  remains 
as  a  mass  of  solid  granules  that  are  discharged  with  or  after  the  gas. 
This  is  a  by-product  of  the  chemical  processes  by  which  the  gas  was 
formed. 

The  entire  mucosa  in  the  cod  is  divided  into  a  large  number  of 
lobules  that  take  an  independent  origin  from  the  basal  tissues.  Between 
these  lobules  the  epithelium  with  its  underlying  connective  tissue  is 
evaginated  into  a  series  of  folds  that  rise  above  and  cover  over  with  their 
edges  the  lobule,  forming  in  this  way  a  common  covering  for  the  entire 
organ,  but  leaving  openings  through  which  the  gas  may  escape  into  the 
bladder. 

This  covering  is  thus  lined  on  both  its  upper  and  its  lower  surfaces 
with  the  undifferentiated  lining  epithelium  of  the  swim-bladder,  and  the 
central  layer  is  composed  of  a  connective  tissue  of  fine  texture  in  which 
run  arteries  and  veins.  The  meaning  of  the  structure  is  not  plain,  and 
requires  further  study.  It  allows  the  gas  to  escape  from  the  gland  by 
the  parting  of  the  sticky  edges  of  its  several  parts,  which  then  drop  back 
into  place. 

Technic.  — There  are  no  special  methods  which  have  been  evolved 
for  the  purpose  of  bringing  out  any  of  the  specific  features  of  the  gas 
epithelium  of  the  gas  gland.  Flemming's  fluid  and  Zenker's  fluid  serve 
to  fix  the  tissues  so  that  all  the  known  structures  may  be  seen  when  the 
sections  have  been  stained  in  iron  haematoxylin. 

LITERATURE 

SCHNEIDER,  K.  C.     "Histologie,"  Jena,  1904,  S.  599. 

REIS,  C.,  und  NUSBAUM,  J.     "Zur  Histologie  derGasdriise  und  s.  w.,"  Anal.  Anz.,  Band 

XXVII,  S.  129,  1905. 
NUSBAUM,  JOSEPH.     "Zur  Histologie  der  tatigen  Gasdruse  und  des  Ovals  bei  den  Teleos- 

tiern.,"  Anat.  Anz.,  Band  XXXI,  Nr.  6. 


CHAPTER    XIX 


THE   EXCRETORY   OR   NEPHRIDIAL   TISSUES 

THE  life  of  all  animals  depends  upon  a  double  process  by  which  com- 
plex tissue  substances  are  built  up  to  be  broken  down  later  in  the  release 
of  some  kind  of  energy.  This  double  process  of  building  up  and  breaking 
down  is  known  as  metabolism.  It  involves  the  securing  and  distribution 
of  food  and  the  collection  and  elimination  of  waste  products  resulting 
from  the  breaking  down  of  tissue  materials.  Tissues  of  alimentation 
and  circulation  take  care  of  the  securing  and  distributing  of  the  food. 
The  nitrogenous  waste  products  of  metabolism  are  poisonous  to  the 
tissues  not  differentiated  for  their  reception,  and  must  be  readily  removed 
from  the  animal  body  or  be  stored 
in  some  tissue  highly  modified  for 
their  reception.  Tissues  of  urine 
excretion,  therefore,  have  evolved 
along  with  the  advance  in  ani- 
mal structure.  In  all  cases,  ex- 
cept in  the  ascidians,  these 
tissues  are  organized  in  such  a 
manner  that  the  nitrogenous 
waste  products  can  get  out  from 
the  body.  These  structures  vary 
much  in  their  complexity. 

Among  the  unicellular  forms 
we  have  seen  that  the  alimenta- 
tion and  distribution  of  food 
was  effected  by  means  of  vacu- 

Oles  Within  the  Cytoplasm  Of  the  FIG.  301.  — Two  infusorians  swimming  in  a  solu- 

Cell.      So     in    the     Same     simple  tion  of  India  ink.     The  matter  discharged  from 

.  the  pulsating  vacuoles  may  be  temporarily  seen 

forms  We  have  VaCUOleS  forming  as  an  irregular  area  of  clear  fluid  next  to  the 

Channels  leading  from  the  endo-  body  and  surrounded  by  the  darkened  water. 

.                  i                 •           <•    i          11  (After  JENNINGS.) 
plasm  to  the  exterior  of  the  cell 

body.  Because  of  their  power  of  rhythmic  contraction  they  are  called 
contracting  vacuoles.  Figure  301  shows  that  these  vacuoles  expel  their 
fluid  contents.  The  fluid  that  the  vacuoles  constantly  throw  out  in  this 

339 


340 


HISTOLOGY 


manner  is  drained  from  the  cell.  Griffiths,  in  1889,  showed  that  this 
fluid  is  charged  with  uric  acid.  This  type  of  excretory  organ  is  common 
to  all  unicellular  organisms  in  which  there  is  a  comparatively  great 
activity.  In  other  words,  where  much  energy  is  displayed,  many  waste 
products  are  formed,  and  an  excretory  cell  structure  has  arisen  to  handle 
them.  Where  the  activity  carried  on  by  the  unicellular  form  is  low  there 
seems  to  be  little  or  no  necessity  for  such  structure,  the  general  surface 
of  the  cell  serving  as  a  medium  for  the  discharge  of  the  waste  products. 
Such  a  contrast  is  seen  in  the  gametes  of  certain  Algae.  The  active 
male  gamete  has  one  or  two  contractile  vacuoles  and  the  passive  female 
gamete  has  none. 

An  example  of  contractile  vacuoles.  — The  contractile  vacuoles  of 
Paramcecium  aurelia  or  Paramcecium  caudatum  are  always  two  in  num- 
ber. Unlike  food  vacuoles,  their  number  is  constant,  and  they  are  sta- 
tionary; also,  they  are  permanent  features  of  the  cell.  Their  inner 


FIG.  302.  —  Individual  of  Paramoecium  caudatum.  Arrows  show  course  of  food  vacuoles  (f.v.*). 
nu.,  nuclei;  con.v.,  contracting  vacuoles,  one  empty  and  one  full;  f.m.,  fecal  matter;  tr., 
discharged  trichocyst.  X  375. 

surface  dips  into  the  endoplasm  and  their  outer  surface  opens  through 
the  ectoplasm  to  the  exterior.  Into  each  contractile  vacuole  a  radiating 
series  of  drainage  channels  lead  (Fig.  302).  These  channels  empty 
their  contents  into  the  contractile  vacuole.  The  channels  are  filled  as 
the  contractile  vacuole  discharges  its  contents.  These  vessels  are  best 
seen  when  filled.  So  the  channels  become  conspicuous  as  the  vacuole 
becomes  indistinct. 

In  the  ccelenterates  there  are  two  factors  which  in  part  account  for 
the  absence  of  specialized  excretory  structures.  First,'  the  activity  of 
these  forms  is  low,  resulting  in  the  formation  of  a  relatively  small  amount 
of  waste  products.  And  secondly,  all  of  these  cells  except  the  very 
passive  mesodermal  cells  have  a  surface  exposure.  It  is  quite  probable, 
therefore,  that  the  surface  exposure  in  the  ccelenterate  body  is  sufficient 
to  remove  the  relatively  small  amount  of  urine  evolved. 

Waste  products  of  the  internal  cells  are  thrown  off  by  these  cells 
into  the  intercellular  mesenchymal  spaces  to  be  passed  to  the  epithelia 


NEPHRIDIAL    TISSUES  •  341 

covering  the  surfaces  of  the  body.  Arriving  at  the  surface  in  a  me- 
dusa, for  example,  these  materials  are  cast  out  into  the  surrounding 
water  through  the  layer  of  epithelial  cells  which  cover  the  surfaces.  In 
this  connection  we  must  bear  in  mind  that  the  intercellular  fluid  carries 
food  materials  as  well  as  waste  materials.  These  and  other  valuable 
materials  are  not  taken  up  by  the  epithelial  cells  to  be  thrown  out  from 
the  body.  We  see,  then,  that  these  cells  can  discriminate  and  select  only 
the  materials  which  must  be  removed  from  the  internal  tissues  and  fluid ; 
with  the  possession  of  this  power  they  become  excretory  or  nephridial 
cells.  These  comparatively  simple  forms  of  excretory  cells  have  not 
been  differentiated  into  a  distinct  excretory  tissue;  they  perform  other 
functions  as  well  as  that  of  excretion. 

In  the  more  active  Metazoa,  where  a  greater  differentiation  has  taken 
place,  tissues  have  been  specialized  that  perform  only  the  function  of 
taking  up  waste  substances  from  collecting  fluids.  These  tissues  have 
the  power  to  take  enough  food  from  the  body  fluids  for  their  own  nour- 
ishment. Except  in  disease  they  take  no  more  of  the  food  materials 
than  this.  On  the  other  hand,  they  select  most  of  the  waste  products 
from  the  collecting  media  and  transfer  them  to  the  exterior.  This  trans- 
ference is  a  vital  and  not  a  mechanical  process.  In  this  respect,  it  differs 
from  the  transfer  or  exchange  of  gases  in  respiratory  tissues. 

Tissues  specialized  for  the  selection  of  urates  from  collecting  and  dis- 
tributing fluids  form  the  nephridia.  In  the  higher  vertebrates,  the  ne- 
phridial tissues  are  assembled  and,  together  with  their  special  blood  and 
nerve  supply  and  connective-tissue  elements,  form  an  excretory  organ, 
the  kidney. 

The  collecting  fluids  are  intercellular  fluid,  ccdomic  fluid,  and  blood. 
Intercellular  fluid  and  ccelomic  fluid  when  associated  with  nephridia 
bathe  them  on  their  proximal  surfaces.  The  blood  supply  is  effected 
in  two  ways.  In  a  few  types  of  animals  the  nephridial  tissues  are  merely 
bathed  in  the  blood.  The  Insecta  furnish  a  good  example  of  this.  This 
first  mode  of  blood  supply  for  nephridial  tissues  is  very  unusual.  Blood 
is  usually  supplied  to  the  nephridial  tissues  through  the  capillaries  of  a 
circulatory  system.  In  the  simplest  tissues  there  is  but  an  ordinary 
capillary  supply.  This  becomes  more  highly  specialized  in  the  higher 
forms.  In  the  vertebrates  there  is  a  general  capillary  supply  as  well  as 
a  terminal  supply.  The  terminal  capillary  structure  is  a  more  or  less 
distorted  plexus  which  is  supported  upon  a  connective-tissue  framework 
at  definite  terminal  regions  of  the  nephridial  tissues.  This  latter  capillary 
structure  is  called  a  glomus. 

The  nephridial  tissues  may  have  various  origins.  We  have  seen 
that  in  the  lowest  Metazoa,  so  far  as  known,  any  surface  cells  may  per- 
form nephridial  functions.  The  higher  forms  of  nephridial  tissues  are 


342  HISTOLOGY 

usually  mesodermal  structures.  They  all  (except  the  ascidians)  have 
effected  a  subsequent  and  secondary  relation  to  the  ectoderm. 

These  tissues  are  always  epithelial.  One  face,  the  proximal  surface, 
of  an  excretory  epithelium  is  directed  toward  the  fluids  from  which 
waste  products  are  being  taken;  the  other  face  forms  the  surface  of  a 
retaining  or  conducting  cavity.  These  tissues  form,  therefore,  sac-like 
or  tubular  organs.  In  the  ascidians  the  renal  epithelium  is  a  vestigial 
coelomic  epithelium.  Into  this  blind  space  waste  products  are  excreted 
and  stored  as  solid  particles.  All  other  nephridial  sacs  or  tubules  de- 
liver the  waste  products  to  the  exterior  through  nephridial  pores  and 
ducts.  In  all  the  simpler  forms  where  the  nephridial  tubules  have  a 
small  lumen,  the  latter  is  intracellular.  In  invertebrates  where  the  lumen 
becomes  large,  and  in  all  vertebrates,  it  is  intercellular. 

The  character  of  the  fluids  with  which  nephridial  tubules  are  func- 
tionally associated,  and  the  manner  in  which  blood  is  brought  to  them, 
has  much  to  do  with  their  structure.  Tubules  associated  with  inter- 
cellular fluid,  simple  lacunar  blood  supply,  or  with  certain  ccelomic 
fluids,  have  usually  two  distinct  regions.  These  two  regions  are  consti- 
tuted by  a  system  of  excretory  tubules  and  by  terminal  excretory  cells, 
of  a  peculiar  type.  In  these  forms  the  lumen  is  usually  intracellular. 
In  the  flat-worms  we  have  such  a  nephridial  system  associated  with  a 
simple  collecting  medium.  In  the  nemerteans  the  nephridia,  though 
associated  with  a  circulating  blood,  are  fundamentally  like  those  of  the 
flat-worms  In  the  rotifers  similar  nephridia  are  associated  with  a 
ccelomic  fluid.  On  the  other  hand,  there  are  no  specialized  end  cells 
in  the  simple  nephridia  of  the  nematods.  This  latter  may  represent 
a  case  of  retrogression.  We  shall  later  examine,  as  an  example  of  these 
simple  nephridia  associated  with  the  simpler  collecting  fluids,  the  ne- 
phridia of  the  tapeworm  commonly  found  in  the  intestine  of  the  robin. 

Tubules  bathed  in  a  blood  supply  are  blind  and  have  a  uniform  struc- 
ture throughout  their  extent.  We  shall  take  as  an  example  of  this  type 
of  nephridia  the  tubule  of  the  insect. 

Certain  forms  in  which  the  nephridia  have  a  coelomic  fluid  from  which 
to  take  excretory  fluids  and  a  general  distribution  of  capillaries  over  their 
walls  have  uniform  structure  throughout  their  extent  and  end  blindly. 
In  connection  with  the  work  they  have  to  do  on  the  ccelomic  fluid 
their  blind  ends  bear  a  group  of  peculiar  terminal  cells  similar  to  the 
terminal  cells  that  in  lower  animals  act  upon  ccelomic  and  other  simple 
collecting  fluids.  Page,  Goodrich,  and  others  described  such  nephridia 
in  various  polychaetes.  We  shall  take  Eulalia  viridis  Mull,  for  an  ex- 
ample. 

Certain  tubules  with  a  general  capillary  supply,  and  having  no  par- 
ticular direct  relation  to  the  ccelomic  fluid,  are  simple  and  undifferen- 


NEPHRIDIAL    TISSUES  343 

tiated  throughout  their  course.  Such  an  example  we  have  in  the  mollusk, 
Sycotypus. 

Tubules  associated  with  a  more  complex  supply  tend  to  become 
differentiated  into  distinct  regions.  In  the  highest  forms  where  the 
capillaries  form  terminal  glomi,  we  have  developed  the  tubule  charac- 
teristic of  all  vertebrates.  We  shall  take  as  examples  the  various  com- 
plex nephridial  tubules  of  Lumbricus,  Homarus,  and  Iguana. 

In  certain  vertebrates  a  group  of  tubules  have  in  common  one 
glomus.  In  these  cases  the  tubules  end  with  more  or  less  expanded 
walls  which  open  into  the  ccelom.  These  open,  expanded  ends  always 
have  strong  cilia  which  are  directed  from  the  ccelom  toward  and  into 
the  lumen  of  the  tubule.  Such  a  terminal  structure  is  known  as  a  ne- 
phrostome and  it  may  remove  solid  as  well  as  fluid  substances  from  the 
ccelomic  fluid.  Among  the  invertebrates  we  shall  take  as  examples 
of  nephrostomes  of  simple,  intermediate,  and  complex  structure  those 
of  Polygordias  neapolitanus,  Perichceta  malamaniensis  Benh.,  and  Lum- 
bricus herculeus,  respectively.  The  nephrostome  of  Ammocotes  will 
be  taken  as  an  example  of  the  vertebrate  nephrostome. 

In  connection  with  the  nephrostomes,  especially  among  the  inverte- 
brates, interesting  accessory  structures  are  developed.  The  general 
ccelomic  epithelium  may  be  considered  in  some  forms  as  excretory  in 
function.  Such  is  clearly  the  case,  according  to  Schaeppe  ('93),  in  the 
ccelom  of  the  annelid,  Ophelia.  The  peritoneum  of  this  worm  elaborates 
chloragogen  in  the  form  of  granules  of  guanin.  In  the  earthworm  the 
peritoneal  epithelium  gives  off  cells  which  become  amoeboid  and  have 
the  power  to  elaborate  excretion  granules.  Among  certain  mollusks 
the  pericardial  epithelium,  which  is  a  ccelomic  structure,  becomes  in- 
tensified by  the  formation  of  glands  known  as  the  pericardial  glands. 
These  give  off  wandering  cells  or  amcebocytes,  which,  after  they  have 
elaborated  their  concretions,  find  their  way  out  through  a  canal  known 
as  the  reno-pericardial  canal.  This  latter  canal,  therefore,  is  functionally, 
if  not  morphologically,  a  nephrostome.  As  examples  of  such  accessory 
renal  structures  we  shall  take  the  ccelomic  epithelium  and  the  wandering 
cells  of  Lumbricus  and  the  wandering  cells  of  the  pericardial  gland  of 
Unio. 

The  function  of  highly  modified  ccelomic  cells  called  chloragogenic 
cells  which  are  found  investing  parts  of  the  alimentary  canal  of  many 
invertebrates  is  yet  to  be  satisfactorily  demonstrated.  According  to 
Ladreyt  ('04)  these  cells  in  Sipunculus  modus  Linn,  excrete  uric  acid. 
For  convenience  they  will  be  described  in  this  connection.  As  an  ex- 
ample we  shall  take  those  found  about  the  intestine  and  large  blood 
vessels  of  Lumbricus. 

Also  as  a  matter  of  convenience  we  shall  here  describe  as  a  type  of 


344 


HISTOLOGY 


excretory  cells  the  calcium  phosphate  cells  which  Barfiirth  so  names 
and  describes  for  Helix.  We  shall  take  as  our  example  those  found  in 
the  hepato-pancreatic  epithelium  of  Mesadon.  It  is  interesting  to  note 
in  this  connection  that  the  concretion  particles  found  in  the  renal  sacs 
of  Nautilus  contain,  according  to  Keferstein,  cal- 
cium phosphate  bodies  and  other  salts,  but  no 
urine. 

In  the  vertebrates  and  some  invertebrates,  well- 
developed  conducting  channels  and  retaining  ves- 
sels for  urine  are  developed.  These  are  the  ureter, 
bladder,  and  urethra. 

The  sweat-glands  elaborate  a  certain  amount 
of  urine  and  other  waste  products.  These  struc- 
tures have  been  considered  in  connection  with 
the  integument.  It  is  of  interest  to  point  out  at 
this  place  that  they  too  are  epithelial  in  charac- 
ter. 

Nephridial  tubules.  The  nephridia  of  the  tape- 
worm of  the  robin.  — The  mesenchyme  of  a  tape- 
worm contains  many  intercellular  spaces  filled  with 
a  body  fluid.  Into  these  passages  waste  solutions 
are  emptied.  This  waste  is  taken  from  the  body 
fluid  by  numerous  excretory  cells  which  deliver 
the  collected  material  to  a  system  of  excretory  cap- 
illaries. The  walls  of  the  capillaries  are  com- 
FIG.  303.— A  flame-ceil  posed  of  very  thin  tile-  or  plate-like  mesenchymal 
cells.  The  ultimate  branches  of  these  capillaries 
bear  the  excretory  cells.  These  cells  have  a  cyto- 
plasmic  body  which  develops  a  collar  (Fig.  303). 
The  collar  forms  the  terminal  part  of  the  wall  of 
the  excretory  capillary.  Into  the  lumen  of  this  collar  the  terminal  cells 
send  a  tuft  of  heavy  cilia  which  in  life  flickers  in  a  manner  that  has  sug- 
gested the  term  "flame"  for  it.  A  large  rounded  nucleus  lies  in  the 
distal  part  of  the  cytoplasm  (Fig.  303,  nu.}.  This  type  of  cell  is  known 
as  a  "flame-cell"  or  solenocyte. 

The  nephridium  common  to  insects  is  a  blind  tubule  known  as  a  M al- 
pighian  tubule.  The  Malpighian  tubule  of  a  caddis-fly  larva  is  simple 
and  unbranched.  The  wall  is  composed  of  two  rows  of  grooved  cells 
that  have  almost  lost  their  individuality  to  form  a  syncytium.  The 
nuclei  on  one  side  of  the  tubule  alternate  with  those  on  the  other.  They 
are  large,  and  have  their  chromatin  uniformly  distributed  as  spherical 
granules.  The  cytoplasm  is  rather  dense  and  homogeneous.  The 
inner  margin  bears  a  less  opaque  striated  cuticula.  The  lumen  is  rela- 


from  the  tapeworm 
found  in  the  robin,  nu., 
nucleus;  c.,  collar;  «., 
neck.  Mass  of  cilia 
inside. 


NEPHRIDIAL    TISSUES 


345 


FIG.  304.  —  Transverse  section  of  a  renal  tubule  of  a  caddis 
larva.  The  tubule  is  formed  (in  transverse  section)  of 
two  cells  which,  because  of  the  alternation  of  their  central 
masses,  show  but  one  nucleus  in  any  given  cross  section. 
The  distal  surfaces  of  the  cells,  where  they  border  on  the 
lumen,  show  a  cuticular  edge,  x  550. 


tively  small.     In  it  excretion  products  are  seen.    The  tubule  is  incased 

in  a  membrana  propria  or  basement  membrane  (Fig.  304).    This  gland  is 

a  modified  and  invagi- 

nated    portion    of    the 

intestinal  epithelium. 
Eulalia  viridis  Miill., 

according  to  Page's  de- 

scription,  possesses  ne- 

phridia  which  are  more 

complex   than   those  of 

the  flat-worm.    The  ne- 

phridium  has  a  conduct- 

ing tubule.    The  lumen 

of  this  tubule  is  intra- 

cellular,  and  its  wall  is  a 

syncytium.    These  latter 

features   are   frequently 

met    with   in   excretion 

tissues.      There    are    a 

few  scattered  cilia  in  the 

lumen.    The  nuclei  are  not  frequent.    The  inner  zone  of  cytoplasm  is 

extremely  finely  granular.    The  outer  zone  is  marked  by  striae  which 

stain  deeply.    This  nephridial  tubule  bears  distally  a  row  of  solenocytes. 

Each  solenocyte  is  represented  by  a  mass  of  cytoplasm  which  is  fused 

with  the  cytoplasm  of  its  neighbor.    The  nucleus  lies  in  this  cytoplasmic 

body.     Each  cytoplasmic  mass  gives  off  a  collar  which  pierces  the  wall 

of  the  tubule  to  empty 
into  its  lumen.  The  wall 
of  this  collar  is  highly 
modified  and  in  being 
less  soluble  in  caustic 
potash  than  cytoplasm 
shows  a  marked  differen- 
tiation. Its  length  is 
twenty  to  twenty-five  mi- 
crons. It  has  a  very  small 
lumen.  Within  this  lumen 
Fage  describes  a  single, 
slender  flagellum  which 

mav  lie  beVOnd    the    collar 

-11  f       i 

into  the  lumen  of  the 
nephridial  tubule.  A  supporting  membrane  is  found  rising  from 
the  nephridial  tubule  and  giving  off  external  flagella  which  probably 


FIG.  305.  —  Inner  end  of  a  renal  tubule  of  the  worm,  Eu- 
lalia.     (After  FACE.) 


346 


HISTOLOGY 


direct  currents  of  the  ccelomic  fluid  toward  the  solenocytes  (Fig.  305); 

such  solenocytes  with  long  and  powerful  cilia  have  also  been  described 

by  Goodrich  ('oo)  for  Aste- 
rope  Candida  Cam.  The  so- 
lenocytes probably  take  up 
waste  materials  from  the 
crelomic  fluids.  The  wall  of 
the  nephridial  tubule  has  a 
blood  capillary  supply.  In 
the  worms  the  chief  amount 
of  excretion  products  is 
taken  from  blood  by  the 
nephridial  walls.  This 
rather  complex  type  of  ne- 
phridium  in  modified  forms 
seems  to  be  present  in  many 
polychaetes. 

The  nephridium  of  Syco- 
typus  is  a  greatly  contorted 
sac.  The  wall  is  furnished 
with  a  connective-tissue  coat 
which  bears  many  blood  ves- 


FIG.  306.  —  Bit  of  renal  epithelium  from  the  gasteropod 
mollusk  Sycolypus.  bl.,  a  blood  sinus  containing 
blood  cells  and  coagulated  blood;  ex.,  large  excretion 
particle  in  cell;  ex.gr.,  a  second  kind  of  excretion  sub- 
stance in  other  cells.  X  1000. 


sels.  These  vessels  open 
into  sinuses  along  the  base- 
ment membrane  of  the 
nephridial  epithelium.  Figure  306  shows  coagulated  blood  serum  con- 
taining blood  lying  next  to  the  basement  membrane.  One  corpuscle 
bears  granules  which  resemble  the  excretion  granules  found  in  certain 
of  the  epithelial  cells  of  the  renal  sac. 

The  epithelium  forms  no  varied  regions  in  the  sac.  It  is  composed 
of  tall,  columnar  cells  measuring  in  height  about  forty-five  or  fifty  microns. 
These  cells  are  all  ciliated.  Oval  nuclei  lie  usually  in  the  basal  third  of 
the  cell.  This  is  not  a  constant  position  for  the  nuclei.  In  some  cases 
nuclei  may  appear  quite  near  the  distal  ends  of  the  cells.  The  various 
nuclei  do  not  stain  uniformly  in  iron  haematoxylin.  The  cytoplasm  is 
alveolar  to  highly  vacuolated.  The  cells  present  two  conditions.  In 
one  condition  the  cell  body  is  rounded,  tending  to  be  cylindrical.  The 
cytoplasm  here  is  greatly  vacuolated,  and  contains  one  or  two  rather  large 
excretion  bodies  or  bears  no  excretion  particles.  In  the  other  condition 
the  cells  become  greatly  flattened  so  that  they  become  somewhat  fan- 
shaped.  These  bear  numerous  refractive,  angular  excretion  particles 
(Fig.  306,  ex.gr.). 

The  nephridial  tubule  of  Lumbricus.  — This  tubule  is  differentiated, 


NEPHRIDIAL    TISSUES 


347 


regions  of  the  upper,  "  narrow  "  part  of  the 
earthworm's  nephridial  tubule.  One  part 
ciliated.  X  870. 


according  to  Benham,  into  five  regions:  (i)  the  narrow  preseptal  tube; 
(2)  "  the  very  long  but  narrow  tube 
in  continuity  with  the  preseptal 
tube" ;  (3)  "the  short,  brownish,  cili- 
ated, middle  tube";  (4)  "the  wide 
muscular  tube  or  duct  which  opens 
to  the  exterior."  Each  of  these  ex- 
cept the  preseptal  tubule  and  the 
short,  brownish,  middle  tubule  is 
thrown  into  a  loop.  The  latter  tu- 
bule communicates  with  the  wide, 

large  tubule  by  means  of  a  distended    Fm  ^  __A  transverse  section  through  two 
part  of  the  nephridial  wall.     This 
distended  region  is  histologically  dif- 
ferentiated  from   both  the  middle, 
brownish  tubule,  and  the  wide,  large  tubule.    This  has  been  named  the 

ampulla  by  Gegenbauer. 

Two  rows  of  cilia  arranged 
in  a  slight  spiral  are  found  in 
the  preseptal  tubule ;  these  cilia 
continue  into  the  first  part  of  the 
very  long,  narrow  tubule,  and 
occur  at  other  points  in  this  lat- 
ter tubule.  The  middle,  brown- 
ish tubule  has  also  two  rows  of 

FIG.  308. — A  transverse  section  of  the  brown  region    cilia  throughout  its  COUrSC.     The 
[the  earthworm's  nephridial  tubule.     X  870.        cytoplasm    of    the    long>     nam)w 

tubule  is  finely  granular.  The  nuclei  are  oval  and  smaller  than  in 
any  other  part  of  the  ne- 
phridium  (Fig.  307).  The 
syncytium  of  the  middle, 
brownish  tubule  is  char- 
acterized by  a  heavy  alve- 
olar  structure.  The 
alveoli  are  most  numer- 
ous near  the  lumen. 
The  cytoplasm  is  more 
abundant  and  the  oval 
nuclei  of  this  tubule  are 

laro-pr    than    thn^p  of    thp     FIG.  309.  — A  transverse  section  of  the  "  ampulla"  of  the 
tne       earthworm's  renal  tubule,   ex.,  excretion  particles.    X  870. 

narrow  tubule.   The  rows 

of  cilia  lie  opposite  each  other  in  the  lumen  (Fig.  308).    The  wall  of 

the   "ampulla"   is   sharply   marked   off   from   that   of  the  brownish, 


348 


HISTOLOGY 


middle  tubule;  on  the  other  hand,  it  merges  slowly  into  the  wall  of 
the  wide,  large  tubule,  so  that  no  sharply  defined  boundary  is  formed. 
«*„„,  Its  cytoplasm  is  dif- 

ferentiated into  an  in- 
ner and  an  outer  layer. 
The  inner  layer  is  ho- 
mogeneous except  for 
excretion  particles 
which  it  may  contain. 
The  outer  layer  of 
cytoplasm  is  distinctly 
striated.  The  striae 
extend  radially  from 
the  periphery  to  the 
inner  layer  of  finely 
granular  cytoplasm. 

The     general    striated 

**•  Q*'  appearance  is  distorted 

FIG.   310.  — Sections  of  earthworm's  tubule  below  "ampulla."        f    n1Prpc    Kv  PYrrptmn 
ex.gr.,  excretion  particle ;  ex. vac.,  excretion  vacuole.     X  870.       aiPiace^     D7  excr( 

bodies    and     vacuoles 

(Fig.  309,  ex.}.  The  nuclei  of  this  region  are  large  and  oval.  They 
have  the  same  general  structure  as  those  seen  in  Figures  308  and  310. 
This  region  of  the  tubule  has  apparently  the  power  to  elaborate  excretion 
products.  The  syncytium  of  the  wide,  large  tubule  lacks  the  inner  layer 
of  "cell  substance"  or  cytoplasm;  otherwise  it  appears  to  be  a  structure 
similar  to  the  "ampulla."  Figure  310  was  taken  from  a  section  near  the 
''ampulla."  Here  excretion  vacuoles  and  an  excretion  body  are  shown, 
ex  vac.  and  ex.  gr.  These  become  less  and  less  frequent  as  the  tubule 
leads  from  the  "ampulla."  The  striae  also  become  gradually  less  distinct 
farther  from  the  ampulla.  The  muscular  duct  is  well  developed.  Its 
diameter  is  about  the  same  as  that  of  the  wide,  large  tubule.  It  is  yet 
uncertain  as  to  whether  its  lumen  is  intra-  or  inter-cellular.  The  cyto- 
plasm of  its  wall  is  difficult  to  demonstrate.  Certain  sections  may  show 
it  bearing  as  many  as  three  nuclei  in  a  single  section;  others  show  no 
nuclei  and  little  cytoplasm.  The  muscles  are  distributed  throughout 
the  wall  of  the  muscular  duct  in  various  directions.  "The  muscular 
duct  penetrates  the  body  wall,  the  muscles  appearing  to  be  continuous 
with  those  of  the  cuticular  layer;  a  slight  invagination  of  the  epidermis 
meets  the  nephridial  lumen,  and  puts  the  latter  into  communication  with 
the  exterior." 

The  closed  nephridium  of  the  Crustacea.  — The  nephridial  tubule 
is  free  of  solenocytes,  but  in  the  higher  forms  it  becomes  com- 
plex. 


NEPHRIDIAL    TISSUES 


349 


In  the  lobster,  Homarus  vulgaris,  the  nephridium  has  an  end-sac  and 
a  middle  convoluted  tubule  both  of  which  eliminate  waste  products. 
The  third  and  terminal  region  has  been  expanded  to  form  a  retaining 
vessel  for  the  fluids  excreted.  The  vessel  delivers  its  contents  through 
a  pore  to  the  exterior.  The  entire  tubule  is  covered  by  a  tunica 
propria  of  connective  tissue,  and  is  lined  with  a  non-ciliated  colum- 
nar epithelium.  The  cells  of  the  end-sac  are  irregular  and  have  the  least 
dense  reticular  cytoplasm.  Distally  they  bear  large  vacuoles  in  which 
excretion  products  are  found.  The  nuclei  are  in  the  basal  third  of  the 
cell  (Fig.  311).  The  epithelium  of  the  convoluted  middle  part  of  the 


FlG.  311.  —  Section  through   parts  of   two  regions  of  lobster's  nephridium.    ex.p.,  excretory 
products;  ex.v.,  excretory  vacuoles.     X  425. 

tubule  is  composed  of  stout  cells  with  very  distinctive  features.  The 
nuclei  are  spherical  and  have  a  conspicuous  reticulum.  There  may  be 
two  nuclei  in  a  single  cell.  The  ends  of  the  cells  bear  a  well-defined 
cuticula.  Beneath  this  there  is  a  zone  of  reticular  cytoplasm  which 
tends  to  be  striated  toward  the  middle  of  the  cell.  The  basal  zone  of 
cytoplasm  is  highly  striated.  These  striae  lie  at  right  angles  to  the  base 
of  the  cell.  Vacuoles  containing  excretion  products  are  found  within  the 
cytoplasm  (Fig.  311,  at  ex.  p.).  When  these  cells  become  highly  active 
the  cytoplasm  at  their  free  ends  becomes  highly  vacuolated,  and  the 
cuticula  becomes  greatly  broken  in  its  contour.  The  cells  lining  the 
storage  region  or  "bladder"  of  the  tubule  lack  a  cuticula.  The  vacu- 
olization  here  is  at  the  basal  end  of  the  cells  instead  of  at  the  free 
ends.  The  marginal  zone  of  cytoplasm  is  dense.  Toward  the  base  a 


350 


HISTOLOGY 


reticular  structure  becomes  more  evident  until  at  the  base  there  are 
many  vacuoles.     The  cells  are  poorly  defined  (Fig.  312). 


FlG.  312.  —  Bit  of  epithelium  from  the  storage  region  or  bladder  of  the  lobster.     X  800. 

The  nephridial  tubule  of  the  lizard,  Iguana.  —  In  this  case  the  termi- 
nal wall  of  each  tubule  is  invaginated  by  a  vascular  plexus  to  form  a 
Malpighian  capsule.  The  capillary  plexus  is  known  as  a  glomus. 


;bl.  co. 


.  t. 


FIG.  313.  —  Section  of  a  glomus  of  the  Iguana's  nephridial  tubule,  bl.ca.,  blood  capillaries 
entirely  filled  with  dense,  nucleated,  red  blood  cells;  ex.c.,  excretory  cells;  c.n.,  capillary 
wall  nucleus;  conn.t.,  connective  tissue;  /.,  lumen  of  tubule  (glomus).  X  1500. 


The  glomus  has  an  afferent  larger  arteriole  which  breaks  up  into  a 
network  of  capillaries.    This  plexus  of  the  arteriole  forms  a  true  rele 


NEPHRIDIAL    TISSUES 


351 


mirabile;  i.e.  it  forms  a  capillary  network  connecting  two  vascular  struc- 
tures of  the  same  character.  The  branching  capillaries,  before  they  leave 
the  glomus,  are  reunited  and  the  efferent  vessel  is  formed,  which  is  also 
an  arteriole  but  slightly  smaller  than  the  afferent  arteriole.  The  capillary 
plexus  is  supported  by  a  slight  framework  of  connective-tissue  cells 
(Fig.  313).  The  glomus  of  Iguana  measures  about  fifty  microns  in 
diameter.  Figure  313  shows  a  section  of  a  glomus  from  the  kidney  of 
this  animal.  The  capillaries  cut  at  various  angles  and  filled  with  blood 
corpuscles  appear  in  this  section  as  dense  bodies  with  a  well-defined  out- 
line. Closely  applied  to  this  contour  at  places  the  flat  nuclei  of  the  cells 
of  the  capillary  wall  are  seen  in  transverse  section  (Fig.  313,  c.n.\ 
These,  except  for  their  position,  resemble  much  the  nuclei  of  the  termi- 
nal epithelium  of  the  tubule.  The  nuclei  of  the  connective -tissue  cells 
are  as  a  rule  smaller  than  those  of  the  cells  of  the  capillary  wall,  and 
of  the  terminal  epithelial  cells  (Fig.  313,  conn.t.). 

The  terminal  epithelium  is  composed  of  thin,  flat,  polyhedral  cells 
which  are  continuous  with  the  main  portion  of  the  tubule.  After  the 
tubule  leaves  the  glomus  its  wall  presents  two  types  of  cells  histologically 
quite  different.  The  tubule  near  the  capsule  is  convoluted.  This  con- 
voluted region  is  the  larger  of  the  two.  Its  diameter  may  be  thirty-five 
to  forty  microns.  The  cells  are  stout  columnar  forms.  In  transverse 
section  they  appear  as  sections  of  truncated  cones  or  pyramids.  They 
measure  twelve  to  fifteen  microns  in  height.  The  inner  margin  of 
the  cytoplasm  has  an  irregular  contour  and  tends  to  be  striated.  Be- 
neath this  there  is  a  zone  of  finely  granular  cytoplasm  which  bears  ex- 
cretion particles.  This  zone  occupies  the  greater  part  of  the  distal  end 
of  the  cell.  The 
basal  or  proximal 
part  of  the  cyto- 
plasm is  more  or 
less  definitely  stri- 
ated and  may  con- 
tain vacuoles.  The 
striae  are  by  no 
means  as  conspicu- 
ous as  those  shown 
in  the  nephridial 
cells  of  a  frog  (see 
Fig.  174).  The  nu- 
clei are  rounded 
and  contain  a  dis- 
tinct reticular  network  with  small,  rounded  chromatin  bodies,  and  one 
large  chromatin  mass  which  may  inclose  a  nucleolus  (Fig.  314). 


FIG.  314.  —  Sections  of  a  thick  and  a  thin  region  of  the  nephridial 
tubule  of  an  Iguana.     X  870. 


352  HISTOLOGY 

The  second  part  of  the  nephridial  tubule  is  narrower  than  the  con- 
voluted portion;  its  diameter  is  about  forty-five  microns.  The  epithe- 
lium of  the  wall  is  composed  of  very  low  columnar  cells  which  tend  to  be 
wider  than  tall.  The  cytoplasm  is  denser  than  that  of  the  convoluted 
portion,  and  the  cells  have  nuclei  slightly  smaller  than  those  of  the  larger 
part  of  the  tubule,  but  which  have  the  same  general  structure.  This 
smaller  portion  of  the  tubule  most  probably  serves  much  less  as  an 
excretory  structure  than  the  convoluted  portion.  The  smaller  part 
of  the  tubule  leads  to  and  empties  into  a  collecting  tubule.  The  latter 
has  an  origin  independent  of  the  nephridial  tubule. 

In  an  embryonic  stage  it  meets  the  nephridial  tubule,  and  its  wall 
coalesces  with  the  wall  of  that  structure.  In  Figure  315  we  have  shown  at 
A  a  place  of  union  between  these  tubules.  The  section  unfortunately 
shows  the  cells  cut  obliquely  so  that  parts  of  cells  cut  above  their  nuclei 
are  seen  lying  to  the  right  of  the  nucleated  portions.  Though  the  section 


FIG.  315.  —  A,  section  through  the  point  of  union  of  the  epithelium  of  a  collecting  tubule  and  a 
nephridial  tubule  in  the  Iguana's  kidney;  B,  some  epithelium  from  the  collecting  tubule. 
X  870. 

is  not  in  this  respect  ideal,  we  may  see  the  nuclei  of  the  nephridial  tubule, 
with  their  characteristic  structure,  interspersed  with  those  of  the  collecting 
tubule.  The  cells  of  the  collecting  tubule  become  taller  as  they  leave 
the  nephridial  tubule.  At  B  in  Figure  315  they  are  shown,  seen  in  pro- 
file. The  cytoplasm  is  finely  granular  and  very  clear  at  the  distal  end. 
At  the  base,  and  in  some  cases  along  the  sides,  a  layer  of  denser  cytoplasm 
occurs.  The  nuclei  are  dense  and  stain  deeply.  They  are  but  eight  or 
nine  microns  in  diameter.  The  nuclei  always  lie  near  the  base  of  the 
cells.  These  cells  closely  resemble  mucous  cells,  while  the  cells  of  the 
nephridial  tubule  are  suggestive  of  serous  cells. 

Examples  of  nephrostomes. — The  nephrostome  of  Polygordius  is 
comparatively  simple.  The  intracellular  lumen  opens  at  the  distal 
end  of  the  nephridial  tubule  into  the  body  cavity.  At  one  margin  of 
this  opening  is  borne  a  tuft  of  cilia  which  is  quite  suggestive  of  the 
"flame"  of  a  "flame-cell"  (Fig.  316). 

In  Perich&ta  malamaniesis  Benham,  the  nephrostome  "consists  of 
eight  or  nine  marginal  cells  set  in  a  circle  around  the  terminal  aperture 


NEPHRIDIAL    TISSUES 


353 


of  the  tubule.  All  the  cells  are  equal  in  size,  and  each  is  ciliated  over  the 
whole  of  the  centrally  directed  face,  the  other  face  being  covered  by 
a  few  ccelomic  epithelial  cells"  (Fig. 


FlG.  316.  —  Section  through  the  long  axis 
of  a  nephrostome  of  Polygordius. 
(After  GOODRICH.) 


In  Lumbricus  herculeus  the  nephro- 
stome has  become  most  complex.  The 
wall  of  the  lumen  of  the  preseptal  tu- 
bule spreads  upon  one  side  to  become 
fan-shaped;  on  the  opposite  side  it 
thins  out  to  become  cleft.  From  this 
cleft  on  each  side  there  diverges  a 
row  of  grooved  cells  which  are  called 
the  centrifugal  cells  (Fig.  318,  cf.\ 
These  cells  meet  a  second  set  of  cells 
known  as  the  centripetal  cells  (Fig.  318, 
cp.}.  The  centripetal  cells,  as  they  are 
removed  from  the  cells  leaving  the  lu- 
men of  the  tube,  become  large  and 
form  nearly  a  complete  ring  of  cells 
about  the  end  of  the  tubule.  The 
cells  of  the  ring  have  been  called  the  marginal  cells.  These  marginal 
cells  are  columnar.  Their  cytoplasm  is  slightly  granular  and  supports 
a  nucleus  near  its  middle.  Between  the  marginal  cells  and  the  cen- 
trifugal cells  there  lies  a  large,  clear,  crescent-shaped  cell  with  a  very 
large  nucleus  lying  at  its  middle.  This  cell  has  been  called  the 
"central  cell"  (Fig.  318,  c.c).  Between  its  inner 
margin  and  the  nephridial  tubule  the  opening  into 
the  nephridial  tubule  is  found.  The  marginal  and 
grooved  cells  form  an  expanded  collecting  appa- 
ratus over  which  many  cilia  are  distributed. 

In  Ammocotes  each  tubule  is  provided  with  a 
ciliated  nephrostome.  The  cilia  are  directed  to- 
ward the  nephridial  tubule.  The  sides  of  the 
nephrostome  are  nearly  parallel,  so  that  the  shape 
of  the  nephrostome  is  but  slightly  like  a  funnel. 
The  cells  forming  its  wall  are  small.  '  They  de- 
crease in  size  as  the  lumen  of  the  tubule  is  ap- 
proached. Likewise  the  cilia  decrease  in  size 
(Fig.  319). 

Examples  of  structures  of  excretion  acces- 
sory to  tubules.  —  In  the  ccelomic  epithelium  of 
Lumbricus  certain  cells  are  found  which  have  elaborated  within  their 
cytoplasm  a  substance  which  in  picro-sublimate  material  has  a  coarse, 


FIG.  317.  —  End  of  a  neph- 
rostome of  Perichata 
malamaniesis .  (After 
BENHAM.) 


354 


HISTOLOGY 


FIG.  318.  —  Nephrostome  of  the  earthworm  Lum- 
bricus  herculeus.     cf. ,  centrifugal  cell ;  c.c.,cen- 


granular  texture  and  a  lemon  color.    They  receive  the  name  of  chlo- 

r  ago  gen  cells.  This  substance  in- 
creases in  bulk  until  the  cytoplasm 
is  for  the  most  part  filled  by  it, 
and  the  nucleus  is  crowded  to 
the  margin  of  the  cell  (Fig.  320, 
A,  B,  and  C).  Such  cells  fre- 
quently, if  not  always,  when 
found  in  the  epithelium,  lie  over 
a  small  capillary  or  space  as  in- 
dicated in  Figure  320,  A,  nv.c. 
These  cells  leave  the  epithelium 
to  become  wandering  cells  or 
amoebocytes.  Figure  320,  C, 
shows  three  such  cells  that  had 
left  the  epithelium  and  were 
found  lying  in  the  ccelom  cling- 
ing to  each  other  as  indicated  in 
the  figure. 

In  Unio  the  epithelium  of  the 

tral  cell ;  cp.,  centripetal  cells  or  marginal  cells,     pericardial    COBlom     in  the    region 
(After  BENHAM.)  rr  .  . '  ? 

of  each  auricle,  is  thrown  into 

folds  to  form  a  gland  known  as  the  pericardial  gland.  The  tissues  of 
this  gland  are  in  inti- 
mate contact  with  the 
walls  of  the  heart,  pene- 
trating the  tissues  of 
the  heart.  By  these 
pericardial  glands,  nu- 
merous wandering  cells 
or  amcebocytes  are 
formed. 

These  amcebocytes 
when  they  begin  their 
activity  are  spherical  to 
oval  in  shape.  Their  cy- 
toplasm is  rather  dense 
and  may  contain  one 
or  more  vacuoles.  They 
are  rather  small  at  this 
stage,  measuring  about 
ten  or  twelve  microns  in  diameter.  The  nucleus  in  the  early  stages  of  ex- 
cretion activity  is  oval,  with  a  diameter  of  about  five  or  six  microns.  It 


ex.  c. 


FIG.  319.  —  Transverse  section  through  the  kidney  of  Ammo- 
cotes.  One  central  glomus  may  be  seen,  covered  with 
excretory  cells  (ex.c.);  cil.t.,  ciliated  part  of  tubule.  (After 
HALLER.) 


NEPHRIDIAL    TISSUES 


355 


contains  distinct  chromatin  granules  of  nearly  uniform  size  (Fig.  321, 
A,  B).  The  excretion  product  first  appears  as  a  small  spherical  mass 
of  dense  homogeneous  sub- 
stance, lying  within  the  cyto- 
plasm near  the  nucleus.  This 
body  continues  to  increase  in 
size  until  it  has  become  a 
large,  dense  sphere  twelve 
microns  or  more  in  diameter. 
About  this  sphere  the  cyto- 
plasm is  applied  as  a  thin  film 
(Fig.  321,  D).  The  nucleus 
is  distorted  and  crowded  to 
one  side  inclosed  in  a  small 
amount  of  cytoplasm.  The 
chromatin  of  the  nucleus  has 
become  less  distinct.  The 
excretion  or  concretion  sphere 
now  breaks  up  into  small 
bodies,  which  are  scattered 
throughout  the  cell  that  has 
somewhat  enlarged  (Fig.  321, 
E  and  JF).  In  the  meantime, 
the  nucleus  shows  a  marked 
tendency  to  divide,  and  fre- 
quently amitosis  is  effected. 
The  nucleus  or  nuclei  finally 
disintegrate,  and  the  cell  has 
completed  its  course  of  excre- 
tory activity.  The  cells  in  this  excretoiy  ceUs"  x  I3°°' 
condition  or  their  fragments  leave  the  gland  through  the  reno-pericar- 
dial  duct  to  the  kidney,  and  from  thence  to  the  exterior. 

The  chloragogenic  cells  of  Lumbricus  are  found  forming  columnar 


FIG.  320.  —  Epithelium  from  the  coelomic  cavity  of 
Lumbricus.  A ,  vertical  section  through  the  epithe- 
lium with  some  of  the  underlying  connective  tissue, 
muscle,  and  blood  vessels;  B,  surface  view  of  three 
cells  and  parts  of  others ;  C,  three  wandering  cells 
in  the  coelomic  cavity;  w.c.,  wandering  cells;  ex.c., 


FIG.  321.  —  Six  stages  of  secretion  elaboration  by  the  amoebocytes  or  "  wan- 
dering cells"  from  the  pericardial  gland  of  Unio.     x  870. 

epithelium  upon  ccelomic  surfaces.    They  are  most  abundant  about  the 
large  blood  vessels  and  the  intestine.    The  typhlasole  is  filled  with  a 


356 


HISTOLOGY 


mass  of  these  cells.  They  are  tall  cells  with  narrow  bases  and  slightly 
expanded  rounded  ends.  The  cells  taken  from  near  the  typhlasole 
measure  one  hundred  microns  or  more  in 
height.  The  cytoplasm  is  highly  alveolar;  in 
the  distal  part  of  the  cell  it  is  the  denser. 
Many  angular  granules  of  chloragogen  are 
usually  held  in  the  cytoplasm.  The  nuclei  are 
small  oval  bodies  lying  near  the  middle  of  the 
cells.  Each  bears  a  single  nucleolus  (Fig.  322). 
The  calcium  phosphate  cells  of  Mesodon  are 
large,  more  or  less  conical  cells,  with  their  bases 
applied  to  the  membrana  propria,  or  basement 
membrane,  of  the  epithelium.  Their  apices 
always  communicate  with  the  lumen  of  the 
hepato-pancreatic  gland.  From  apex  to  base 
they  measure  thirty  to  thirty-five  microns.  The 
cells  are  isolated  and  surrounded  on  all  sides 
by  the  hepato-pancreatic  cells  (Fig.  323). 

The  ureter  and  bladder  serve  chiefly  as  con- 
ducting and  retaining  structures.  Because  of 
this  function  their  epithelium  is  compact  and 
stratified.  The  basal  layers  of  cells  in  these  retaining  epithelia  tend  to 
be  columnar.  These  support  a  superficial  layer  of  flattened  cells  which 
lie  parallel  to  the  surface  of  the  bladder  wall.  The  cytoplasm  of  the  cells 
is  dense.  The  nuclei  are  rounded  and  centrally  placed  (Fig.  324). 

Beneath  the  epithelium  in  both  the  ureter  and  bladder  is  a  connective 
tissue,  tunica  propria,  supplied  with  scattered  elastic  fibers,  lymphatics, 
and  blood  vessels.  In  both  the  ureter  and  bladder  there  is  developed 
an  outer  coat,  tunica  mus- 
cularis.  This  also  has  a 
connective-tissue  frame- 
work in  which  smooth 
muscle  fibers  are  distrib- 
uted in  one,  two,  or  three 
layers. 

Technic.  —  We  have 
here  to  do  with  a  very 
easy  tissue  to  cut  and 


FIG.  322.  —  Several  chlora- 
gogen cells  from  around  the 
intestine  of  the  earthworm. 
X  650. 


FIG.  323.  —  Cells  from  the  digestive  gland  of  Mesodon 
(Helix),  cal.ph.c.,  calcium  phosphate  cell.  Others  are 
hepato-pancreatic  cells,  b.m.,  basement  membrane,  on 
which  lies  a  narrow  connective-tissue  nucleus.  X  970. 


stain     by     the     ordinary 

methods.     Sometimes  the 

vertebrate    kidney,  as   in 

the  mammals,  and  in  Petromyzon,  will  prove  refractory  and  get  brittle, 

but  a  second  attempt  with  a  short  fixation  and  careful  treatment  in  the 


NEPHRIDIAL    TISSUES  357 

water  bath  will  usually  give  the  desired  results.  If  this  fail,  one  can  be 
sure  of  getting  sections  by  the  celloidin  method,  in  which  the  difficulty 
of  brittleness  does  not  emerge. 

In  addition  to  the  sections,  it  is  very  desirable 
to  control  the  results  of  sectioning  by  a  careful 
study  of  teased  specimens.  This  method  is 
vastly  preferable  to  the  use  of  reconstruction 
methods  in  the  working  out  of  long  tubules,  etc. 
One  should  complete  no  comparative  study  of 
the  renal  tissues  without  watching  the  live  kid- 
neys of  a  small  annelid  worm  under  the  micro- 
scope. Other  nephridia  may  also  be  studied  in  FIG.  324.— TWO  basal  cells 
this  way,  in  or  out  of  the  body.  !±£S£  S££ 

To  determine  the  nature  of  a  suspected  ne-       striated  epithelium  which 

phridial  Cell  Or  tissue  it  is  Often  possible  tO  Watch         lines  the  bladder  of  a  mam- 
mal.   (After  KOELLIKER.) 

the  cells  excrete  some  foreign  material,  as  car- 
mine, which  has  been  placed  in  the  blood  or  body  cavity  of  the  organ- 
ism.   This  may  even  be  seen  in  sections  taken  the  proper  time  after 
the  material  is  injected. 

LITERATURE 

AWERINZEW,  S.     "Beitrage  zur  Kenntnis  der  maim  Rhizopoden,"  Mitth.  Zool.  Slat,  zu 

Neapl.,  Band  XVI,  S.  349-364,  1903. 
JENNINGS,  H.  S.     "A  Method  of  Demonstrating  the  External  Discharge  of  the  Contractile 

Vacuole,"  Zool.  Anz.,  Band  XXVII,  pp.  656-658,  i  fig.,  1904. 
BARTHELS,  PH.     "Notiz  iiber  die  Excretion  der  Holothurien,"  Z.  Anz.,  Jahrg.  18. 
COTTE,  JULES.     "  Excretion  of  Sponges,"  Bull.  Scient.  France,  Belgique,  T.  38,  pp.  420- 

573,  9  fig.,  1904. 
BENHAM,  W.  B.     "The  Nephridium  of  Lumbricus  and  its  Blood  Supply,"  Quart.  Journ. 

Mic.  Sc.  (2),  Vol.  XXXII,  p.  293,  1891. 
FACE,   Louis.     "Recherches  sur  les  organes  segmentaires  des  Annelides  Polychetes," 

Ann.  Sc.  Nat.  Zool.  (9),  T.  3,  pp.  261-410,  2  pis.,  52  fig.,  1906. 
GOODRICH,  E.     "On  the  Nephridia  of  the  Polychzetes,"  Part  i,  Q.J.M.S.,  Vol.  XL,  1897 

(and  other  papers  in  same  Journal). 
HERNEBEL,  M.  A.     "Observations  sur  le  role  des  Amcebocytes  dans  le  coelome  d'un  anne- 

lide,"  Ann.  Inst.  Pasteur,  T.  17,  pp.  449-461,  2  pis.,  1903. 
CUENOT,  L.     "L'excretion  chez  les  mollusques,"  Arch,  de  Biol.,  T.  16,  1899. 
BOURNE,  G.  C.     "On  the  Structure  of  ^Enigma  (znigmata,"  Quart.  J.  Mic.  Sc.,  N.S.,  N. 

202,  May,  1907,  Sec.  p.  274. 
GRIFFEN,  L.  E.     "Renal  Sacs  of  Nautilus,"  Memoirs  of  the  Biol.  Lab.,  Johns  Hopkins 

University,  Vol.  V,  p.  165,  1903. 
HENSCHEN,  F.    "Zur  Kenntnis  der  blasenformigen  Sekretion,"  Anat.  Hefte,  Band  XXVI, 

s-  573-594,  2  fig.,  9,  1904. 
BRENTY,  L.     "Contribution  a  1'etude  de  1'excretion  chez  les  Arthropods,"  Arch.  Biol., 

T.  20,  p.  217-422,  3  pis.,  1903. 
HOFFMANN,  R.  W.     "  tiber  den  Ventraltubus  von  Tomocerus  plumbeus,  Z.  und  seine  Bezie- 

hungen  zu  den  grossen  unteren  Kopfdrvisen,"  Zool.  Anz.,  Band  XXVIII,  S.  87-116, 

19  fig.,  9,  1904. 
REGAUD,  CL.     "Demonstration  relative  au  segment  cile  du  rein  de  Petromyzon,"  Verh. 

Anat.  Ges.,  18  Vers.,  p.  181,  1904. 


CHAPTER   XX 
THE   INTEGUMENT,   TISSUES   OF  MECHANICAL  PROTECTION 

THE  integument  consists  of  the  exposed  outer  portions  of  the  covering 
epithelium  of  the  body,  added  to,  in  nearly  all  cases,  by  certain  connec- 
tive tissue,  muscle,  and  other  cells  that  are  located  in  its  neighborhood. 
By  "exposed  outer"  is  meant  such  portions  as  are  directly  exposed  to 
the  medium  in  which  the  animal  lives  (air  or  water).  The  similarly 
constructed  coverings  of  many  internal  cavities  into  which  water  or 
air  are  brought  for  respiratory  or  other  purposes  will  be  considered  as 
a  form  of  integument  and  not  further  differentiated  from  the  former  in 
this  work  except  in  so  far  as  real  differences  exist  between  the  two. 

On  account  of  the  superficial  position  of  an  integument,  its  functions 
are  most  numerous.  In  the  simplest  and  smallest  organisms  it  performs 
most  of  the  functions  of  the  body.  In  the  larger  and  more  complicated 
creatures,  many  functions  are  still  performed  by  portions  of  this  surface 
layer,  but  these  portions  are  removed  from  the  surface  by  invagination 
to  internal  positions  in  the  body,  this  happening  in  various  degrees 
according  to  the  conditions. 

Although  the  surfaces  used  to  perform  the  principal  functions  of  the 
body,  in  the  majority  of  organisms,  have  been  removed  from  the  outer 
integument,  a  number  that  cannot  be  performed  internally  have  remained 
in  it,  and  in  addition  to  these  are  found  minor  duplications  of  some  of 
those  that  have  been  removed  to  the  inside.  These  latter  have  often 
acquired  some  secondary  use  in  the  integument. 

With  these  functions  as  a  chief  basis  for  classification  and  with  the 
aid  of  ontogenetic  origins,  we  may,  for  convenience,  classify  the  integu- 
mentary tissues  as  tissues  of:  — 

A.  Mechanical  protection  (and  adornment). 

B.  Offensive  mechanical  protection  and  production  of  poisons. 

C.  Lubrication  and  cleansing. 

D.  The  production  of  attractive  and  repulsive  odors. 

E.  Adhesion  and  spinning. 

Of  the  functions  of  the  integument,  mechanical  protection  is  one  of 
those  that  belongs  peculiarly  to  it.  We  mean  by  the  term  a  protection 
against  pressure,  abrasion,  and  the  entrance  of  needless  or  harmful  fluids 

358 


INTEGUMENT 


359 


or  other  substances.  Naturally,  this  work  cannot  be  directly  shared  in 
by  any  other  tissue  in  the  body.  The  surface  cells  must  perform  it, 
assisted  indirectly,  in  many  cases,  by  the  cells  that  lie  next  to  and  inside 
of  them.  The  simplest  way  in  which  this  work  is  done  is  by  some  kind 
of  stiffening  and  hardening  of  the  cells  of  the  epithelium.  We  shall 
study  this  method  both  in  columnar  and  stratified  epithelia. 

In  the  simplest  columnar  epithelia,  this  mechanical  function  is  made 
evident  by  the  lengthening  of  the  cell^  the  formation  of  stiff  fibrils  in  its 
cytoplasm  (to  offset  pressure),  the  thickening  of  its  outer  border  (to  resist 
abrasion),  together  with  the  formation  of  terminal  bars  (to  prevent  the 
entrance  of  harmful  or  needless  fluids). 

These  structures  may  be  found  performing  their  duties  alone,  as 
in  the  covering  epithelium  of  some  flat  worms  (Planocera),  or  in  com- 
bination with  many  of  the  accessory  structures 
to  be  described  hereafter.  They  can  be  studied 
in  a  large  number  of  the  lower  animals,  and 
we  shall  first  examine  them  as  seen  in  the  integ- 
ument of  a  turbellarian  worm,  Planocera  fo- 
lium (Fig.  325). 

Here  two  of  the  features  mentioned  above  are 
most  excellently  shown.  The  supporting  func- 
tion is  performed  by  a  series  of  stiff  fibrils  that 
extend  from  the  proximal  surface  of  the  cell  to 
its  distal  surface.  Several  granules  may  be  seen 
on  each  of  the  fibrils  near  its  outer  end,  and  at 
the  point  where  it  leaves  the  surface  a  larger 
granule  is  placed.  The  fibril  is  apparently  con- 
tinued directly  through  this  granule  to  form  one 
of  the  cilia  that,  together  with  the  other  cilia 
belonging  to  the  same  cell  and  to  the  other  cells, 
cover  most  of  the  body. 

Thus  the  supporting  fiber  is  evidently  used  for  at  least  one  other  pur- 
pose than  the  more  passive  support  of  the  cell.  It  is  also  used  to  support 
the  moving  cilium,  and  it  is  possible  that  it  also  has  structural  features 
that  play  a  part  in  moving  the  cilium.  It  may  even  be  conceived  that 
other  cell-organs  have  important  structural  relations  with  the  fibril.  Such 
ideas  will  not  detract,  however,  from  the  conception  of  it  as  a  cell-organ 
of  mechanical  support. 

The  fibril,  so  plainly  seen  as  a  straight  support  in  this  cell,  is  also  found 
in  many  other  epithelial  cells  throughout  the  different  epithelia.  These 
fibrils  may  be  very  slight,  branched,  net-forming,  and  otherwise  variable, 
and  in  some  cases  hard  to  distinguish  from  fibrils  of  a  totally  different 
nature. 


FIG.  325.  —  Protective  and 
ciliated  epithelial  cell  from 
the  body  surface  of  a  flat 
worm,  Planocera.  (After 
SCHNEIDER.) 


360 


HISTOLOGY 


The  function  of  resisting  the  entrance  of  foreign  and  unnecessary  fluids 
is  performed  by  two  structures,  the  outer  surface  of  the  cell  itself  through 
the  physiological  processes  that  take  place  in  or  near  that  surface,  and 
also  by  a  series  of  terminal  bars  or  "  schlussleisten "  found  between  the 
edges  of  the  outer  surfaces  of  the  cells  (see  Fig.  47).  These  structures  are 
rodlike  and  double,  the  two  parts  being  closely  applied  and  cemented  to 
each  other.  Each  half  is  structurally  a  part  of  the  lateral  cell-wall,  and 
even  when  the  lateral  walls  are  separated  one  from  the  other,  they  re- 
main connected  by  the  closing-plates  until  the  separating  forces  become 
very  much  stronger. 

Any  special  structural  device  developed  to  perform  the  third  protec- 
tive function  of  resisting  abrasion  is  not  well  shown  in  this  example  on 
account  of  the  presence  of  the  cilia,  which  render  it  unnecessary  and 
impossible.  Such  a  device  usually  appears  as  a  thickened  and  hardened 
portion  of  the  distal  end  of  the  cell,  forming  a  platelike  structure  on  the 
end  of  the  cell.  It  may  be  seen  to  advantage  in  the  digestive  cells  of 

the  small  intestine  of  vertebrates  and 
in  other  places  as  on  the  outer  epithe- 
lium of  Amphioxus  (Fig.  326). 

The  next  step  in  the  development 
of  the  three  functions  under  discus- 
sion, in  a  columnar  form  of  epithelium, 
is  the  formation  by  the  cells  of  an 
external  and  extracellular  layer  of 
some  substance  that  will  perform  the 
functions  better  than  the  cells  could 
themselves  do.  Such  an  organ  is  the 
cuticle,  which  is  formed  jointly  by  all 
the  cells  as  a  continuous  layer  covering 
the  epithelium.  In  some  cuticles,  the  portion  formed  by  each  single 
cell  can  be  distinguished  from  that  formed  by  the  surrounding  cells,  but 
in  most  it  cannot. 

The  cuticle  is  laid  down  by  the  cells  in  layers  in  most  cases,  or  ap- 
parently as  a  single  layer  in  some.  It  sometimes  contains  fibers,  and  in 
many  cases  it  bears  various  points,  knobs,  and  other  structures  on  its 
surface.  These  structures  may  be  the  product  of  one  cell  or  of  many 
cells.  Various  openings,  usually  of  small  size,  serve  as  a  means  of  exit 
for  the  secretions  of  glands  and  the  cilia  and  nerve-endings  of  other  cells 
as  well  as  to  permit  odors  to  reach  some  olfactory  cells. 

The  cuticle  is  often  strengthened  by  the  addition  of  mineral  salts 
and  the  addition  of  foreign  substances  to  the  exterior.  It  is  also  renewed 
in  most  forms  either  by  small  parts  at  a  time  (leech),  or  by  a  process  of 
entire  shedding  of  the  whole  structure  at  once  (lobster).  In  this  case 


FIG.  326.  —  Protective  epithelial  cells  from 
the  body  surface  of  Amphioxus.  The 
two  kinds  of  cells  represent  two  kinds  of 
fixation  as  well,  probably,  as  different 
physiological  conditions.  (First  pair  of 
cells  after  SCHNEIDER.)  x  1000. 


INTEGUMENT 


361 


its  place  is  taken  by  a  new  cuticle  that  is  usually  formed  before  the  old 
one  is  removed. 

The  substance  of  the  cuticle  is  an  organic  material  called  keratin, 
which  varies  somewhat  in  the  different  forms.  It  is,  in  some  cases,  said 
to  be  made  of  cellulose.  The  cuticle  found  on  many  worms  is  a  simple 
type,  of  moderate  development,  and  that  of  the  earthworm  will  do  for 
examination. 

The  cuticle  of  the  earthworm  is  a  layer  of  material  about  one  eighth 
or  one  tenth  of  the  thickness  of  the  layer  of  epithelial  cells.  Its  structure 
does  not  appear  to  advantage  in  a  section,  and  it  is  best  studied  in  a  piece 
of  the  layer  that  has  been  torn  or  macerated  off  in  alcohol  of  30  per 
cent  (Fig.  327).  Here  it  is  seen  that  the  whole  structure  of  this  organ 
consists  of  two  parallel  series  of 
fibrils,  each  set  lying  at  right  an- 
gles to  the  other,  and  all  flat- 
tened together  by  a  cement  sub- 
stance that  fills  all  the  interstices. 
At  somewhat  regular  intervals 
the  fibrils  are  pressed  somewhat 
apart  by  a  greater  amount  of  the 
cement  substance,  and  through 


FIG.  327.  —  Superficial  view  of  cuticle  from  an 
earthworm.     X  1500. 


this  thickened  cement  may  be 
seen  a  fine  pore  passing  through 
the  entire  layer.  This  pore  is  seen  in  transverse  sections  of  the  cuticle 
in  situ  on  the  epithelium,  to  provide  a  passage  for  the  secretion  of  one 
of  the  many  unicellular  mucous  gland-cells  that  are  to  be  found  at  nu- 
merous intervals  on  the  surface  of  the  body. 

Certain  regions  of  the  cuticle  show  spots  that  provide  many  similar 
pores  set  closely  together.  Again,  the  section  of  the  entire  integument 
will  show  that  these  are  the  passages  through  which  the  perceptory 
endings  of  a  number  of  nerve  cells  come  to  the  surface  where  they  will 
be  in  contact  with  the  outer  conditions. 

The  fibrils  that  make  up  the  bulk  of  the  cuticle  are  straight  and  par- 
allel, and  run,  as  has  been  said,  in  two  series,  the  strands  of  each  series 
being  at  right  angles  to  the  strands  of  the  other.  Both  series  are  sym- 
metrically oblique  to  the  main  axis  of  the  animal's  body,  and  are  thus  at 
an  angle  of  forty-five  degrees  to  it.  The  fibrils  are  thought  to  pass  from 
one  surface  to  the  other,  and  to  interweave.  It  is  puzzling  to  understand 
how  the  epithelial  cells  that  are  arranged  all  over  the  surface  of  the  body, 
and  in  no  particular  order,  are  able  to  cooperate  in  such  a  manner  as  to 
lay  down  or  form  such  long,  straight,  and  parallel  fibers  in  two  distinct 
sets. 

The  exact  method  by  which  the  cuticle  is  laid  down  by  the  cells  has 


362 


HISTOLOGY 


not  been  especially  worked  out  in  the  earthworm.  Also  the  renewal 
processes  which  must  take  place  to  repair  the  loss  by  attrition  in  this 
hard-burrowing  animal  are  not  understood.  For  methods  of  cuticle 
renewal,  see  the  following  description  of  this  process  in  the  Crustacea. 
The  two  have,  doubtless,  much  that  is  similar. 

The  lobster  is  also  an  animal  that  has  an  epithelial  layer  of  cells  on 
the  outside  of  its  body  and  a  cuticle  covering  these  cells.    The  columnar 

cells  composing  this  epithe- 
lium are  large  and  well 
formed,  and  vary  much  as 
to  length  and  the  develop- 
ment of  their  characteristic 
organs,  which  are  the  same, 
however,  and  easily  distin- 
guished wherever  seen.  The 
supporting  fibrils  are  partic- 
ularly well  seen,  and  are 
very  instructive  because  of 
the  fact  that  they  are  to  be 
divided  into  two  groups  ac- 
cording to  whether  muscles 
are  attached  to  the  cells  to 
which  they  belong  or  not. 
In  Figure  328  a  portion  of 
the  epithelium  of  a  lobster 
is  represented  with  the 
cuticle  lying  on  its  upper 
surface  and  two  kinds  of 
supporting  fibrils  repre- 
sented in  the  cells.  The 
cells  on  the  left  are  from 
a  region  where  the  fibers 
of  a  muscle  are  attached, 
and  the  fibrils  of  this  mus- 
cle can  be  seen  so  closely 

FIG.  32 8. -Portion  of  the  new  integument  of  a  lobster,  Connected  ^  with  the  fibrils 
Homarus.  conn.t.nu.,  connective-tissue  nucleus;  bl.c.,  of  the  epithelial  Cell  that 
blood  cells;  mus.c.,  muscle  cells.  .-,  .  i  ,Jjrprf  ron 

tinuations  of  them.  The  supporting  fibrils  are,  in  this  case,  made 
strong  and  straight  so  that  they  can  bear  the  strain  of  the  contraction  of 
the  muscle.  In  the  adjoining  half  of  the  epithelium,  in  the  right  of  the 
illustration,  it  can  be  distinctly  seen  that  the  supporting  fibrils  of  the  cells 
are  not  called  on  to  withstand  any  such  strain  because  of  the  blood  space 


INTEGUMENT 


363 


that  bounds  their  proximal  surfaces,  and  here,  accordingly,  we  see  no  such 
strength  and  straightness  of  the  fibrils.  They  are  somewhat  branched 
in  these  cells  and  of  much  more  delicate  for- 
mation. 

This  epithelium  is  also  interesting  because 
of  the  fact  that  connective-tissue  cells  have 
wandered  among  the  bases  of  the  cells,  and 
because  it  is  an  example  of  an  epithelium 
without  a  basement  membrane.  The  lateral 
boundaries  of  the  cells  are  very  thin  and  very 
difficult  to  see.  As  in  the  case  of  other  epi- 
thelia  that  are  used  to  form  a  shell  of  lime, 
the  nuclei  of  the  cells  are  nearer  the  distal  end 
of  the  cell  than  those  of  the  majority  of  other 
epithelial  cells.  This  latter  fact  is  particularly 
true  of  the  plecypod  mollusks. 

The  cuticle  that  we  find  in  the  earthworm 
is  represented  in  the  lobster  by  a  thick  struc- 
ture, the  shell,  which  is  a  real  cuticle  of  or- 
ganic material,  stiffened  and  hardened  by  the 
deposition  of  salts  of  lime.  The  organic  base 
of  the  shell  consists  of  a  square-meshed  reticu- 
lum built  up  of  fibrils  (see  Fig.  329).  In  the 
meshes  of  this  reticulum  is  a  more  weakly  devel- 
oped groundwork  of  organic  substance,  the 
whole  plastic  structure  being  combined  with 
the  lime  salts  much  as  it  is  in  bone. 

The  specimen  that  we  are  studying  is  a 
section  of  the  integument  of  a  lobster  that 
had  just  shed  its  shell,  and  had  not  yet  had 
the  lime  salts  deposited  in  the  new  one. 
Thus  it  may  be  assumed  that  we  have  the 
best  view  obtainable  of  the  organic  structure 
of  the  shell,  and  it  can  be  seen  that  it  is 
stratified,  with  the  strata  somewhat  thicker 
and  less  distinct  the  farther  they  lie  from 
the  cells.  The  darker  parts  of  the  substance 
that  are  thinnest,  and  which  serve  to  separate  the  strata  from  each 
other,  are  the  horizontal  lines  and  meshes  of  the  reticulum,  and  are  much 
easier  to  distinguish  than  the  vertical  lines.  The  layers  thus  seen  are 
to  be  grouped  into  a  number  of  regions  in  the  thickness  of  the  shell.  The 
best  grouping  of  these  layers  seems  to  be  into  an  outer  and  very  thin  region 
that  is  denser  and  darker  than  the  rest  of  the  shell,  a  somewhat  thicker 


groundwork  of  a  lobster's 
shell.  The  figure  is  discon- 
tinuous at  x,  and  incomplete 
on  the  upper(  surface  for 
want  of  room.  (After 
SCHNEIDER.) 


364  HISTOLOGY 

middle  region  that  contains  the  pigment,  and  an  inner  or  principal  region 
that  seems  to  serve  as  the  chief  means  of  mechanical  support.  The 
strata  of  the  inner  region  are  hardest  in  the  outer  portion,  and  become 
softer  as  they  are  examined  nearer  the  cells.  The  innermost  is  but 
weakly  calcined. 

In  general,  the  cuticle,  in  its  office  of  mechanical  protection,  is  often 
found  to  bear  modified  portions,  as  lumps,  ridges,  knobs,  hairs,  spines, 
etc.,  of  complicated  patterns  and  varying  sizes.  The  simplest  form 
of  this  modification  is  where  a  cuticle-like  structure  that  ordinarily  ex- 
tends in  an  unbroken  sheet  over  the  surface  is  more  highly  developed 
in  certain  locations,  forming  isolated  areas,  rods,  cones,  spikes,  or  one 
or  another  of  an  almost  infinite  variety  of  structures  scattered  more  or 
less  regularly  over  the  surface  of  the  integument.  In  all  these  cases  the 
structures  in  question  are  formed  like  other  cuticle  by  the  action  of  the 
epidermal  cells.  Sometimes  one,  sometimes  many,  of  these  cells  form 
each  such  a  structure,  and  frequently  the  structure  acquires  a  secondary 
use.  The  strange  cuticle-like  structure  found  in  the  gizzard  of  the  bird, 
the  stomach-jaw  of  the  Arthropoda,  and  the  other  organs  of  mastication 
of  a  cuticular  nature  have  been  treated  of  under  the  tissues  of  mastica- 
tion (Chapter  XV). 

Turning  our  attention  again  to  protection  against  abrasion  by  an 
epithelium,  we  find  that  stratification  (Chapter  VI)  is,  in  its  essential  fea- 
tures, a  highly  developed  means  of  mechanical  protection.  The  struc- 
tural devices  that  a  stratified  epithelium  develops  to  accomplish  its  ends 
are  much  the  same  as  in  the  simple  epithelium.  They  are  simply  modi- 
fied to  meet  the  different  structure  of  this  form  of  epidermis. 

A  cuticle  is  not  formed  as  an  extracellular  structure  in  the  stratified 
forms  of  epithelia.  But  the  outer  cells  of  the  epithelium  are  modified 
so  greatly  that  they  form  layers  of  different  structure  and  consistency 
that  are  admirably  adapted  to  all  purposes  that  a  cuticle  could  fulfill. 
Only  in  this  case  it  is  the  cells  themselves  that  perform  the  work,  and  not 
an  extracellular  material  that  they  have  formed.  The  similarity  of  the 
two  processes  is  much  heightened  by  the  fact  that  the  cells  that  do  this 
duty  are  dead  themselves  and  might  be  compared  very  closely  with  a 
cuticle  from  the  standpoint  of  function,  far  removed  as  they  are  from  it 
in  their  origin.  This  form  of  outer  protection  is  also  somewhat  more 
convenient  than  the  cuticle  of  a  simple  epithelium  because  it  does  not 
have  to  be  shed  at  intervals,  except  in  a  very  few  cases,  but  is  continually 
and  gradually  dropped  and  renewed.  For  a  very  simple  sort  of  strati- 
fication see  Chapter  VI,  where  this  is  illustrated  in  a  chaetognath  worm 
Sagitta.  An  example  of  this  principle,  carried  to  a  large  degree  of  effi- 
ciency without  any  great  specialization,  is  furnished  in  the  skin  of  man. 
This  is  pictured  in  Figure  330.  A  small  region  of  this  epidermis  is  here 


INTEGUMENT 


365 


shown  resting  on  a  portion  of  the  underlying  mesodermal  tissues  called 
the  derma.  This  derma  is  a  part  of  the  skin,  being  especially  set  apart 
from  the  more  central 
tissues  and  strengthened 
to  afford  a  strong  and 
elastic  bed  for  the  epi- 
thelium. A  basal  mem- 
brane is  present,  and 
upon  its  slightly  curved 
surface  rests  the  basal 
layer  of  the  epithelium. 
This  layer  is  made  up 
of  a  very  perfect  layer 
of  slightly  columnar 
cells  whose  nuclei  are 
oval,  probably  on  ac- 
count of  their  some- 
what crowded  condition. 
They  lie  in  the  cell  at 
some  little  distance  from 
the  basal  membrane, 
and  are  frequently  met 
with  in  mitotic  division. 

The  many  cells  de- 
rived from  this  basal 
layer  lie  above  it  in  a  far 
thicker  layer  of  cells 
that  takes  up  about  one 
half  of  the  entire  thickness  of  the  epidermis.  This  is  known  as  the 
stratum  germinativum.  Most  of  the  cells  in  this  layer  show  evidences 
of  an  amitotic  division,  a  terminal  process  in  the  life  of  the  cell.  The 
nucleus  and  cell  both  grow  in  size  until,  at  the  outer  boundary  of  the 
layer,  they  are  transformed  by  a  deposit  of  granular  matter  into  the 
cells  of  a  thin  granular  layer,  known  as  the  stratum  granulosum. 
This  layer  is  about  two  cells  thick,  and  its  flattened  nuclei  show  signs  of 
degeneration.  The  layer  stains  rather  deeply. 

Above  this  stratum  the  cells  are  changed,  first  into  a  homogeneous 
and  non-staining  stratum  lucidum,  and  then  by  a  stratification  and  harden- 
ing as  well  as  flattening,  into  an  outer  stratum  corneum.  Both  of  these 
two  last  layers  are  dead  cells  which  are  toughened  and  developed  to  act 
as  buffers  between  outer  abrasion  and  the  delicate  and  living  tissues  be- 
neath them.  As  fast  as  any  of  these  cells  are  rubbed  off,  new  ones  are 
added  from  beneath.  A  cell  from  this  outer  layer  is  large  and  thin  and 


FlG.  330.  —  Portion  of  a  vertical  section  through  the  epider- 
mis of  man.  c.,  part  of  the  underlying  corium;  b.,  basal 
layer  of  the  epithelium,  separated  from  corium  by  the  thin 
basement  membrane;  g.,  stratum  germinativum;  gr., 
stratum  granulosum;  /.,  stratum  lucidum;  cor.,  stratum 
corneum.  X  350. 


366  HISTOLOGY 

has  a  flat,  round,  and  non-staining  nucleus.  Its  strength  comes  from 
a  deposit  of  a  substance  called  first  kerato-hyalin,  which  then  changes 
into  eleidin,  and  that  into  pareleidin. 

Just  as  the  cuticle  of  the  columnar  epithelium  is  developed  into  a 
variety  of  organs,  so  also  is  the  outer  protective  layer  of  toughened  dead 
cells  in  the  stratified  epithelium  developed  into  a  vast  number  of  struc- 
tures that  are  used  for  a  variety  of  purposes.  In  a  very  general  way 
it  may  be  said  that  these  structures  represent  and  are  made  out  of  the 
same  kind  of  outer  cells  that  cover  the  surface  of  the  stratified  epithelium. 
They  become  so  hardened  and  fitted  together,  however,  that  it  is  difficult 
to  distinguish  them  as  cells.  These  structures  are  usually  developed  from 
a  portion  of  the  epithelium  that  has  been  invaginated. 

One  of  the  commonest  and  simplest  forms  of  this  structure  is  an 
evagination,  and  is  to  be  found  on  the  tongue  of  most  mammals,  espe- 
cially on  those  which  eat  living  prey.  The  upper  surface  of  the  tongue 
is  evaginated  into  a  series  of  close-set  papillae  covered  with  their  strati- 
fied epithelium.  In  the  hollows  this  epithelium  is  thin  and  soft.  On  the 
pointed  tips  it  is  immensely  thick  and  strong,  and  is  so  hardened  and 
compacted  that  the  animal  can  use  its  tongue  as  a  rasp  to  scrape  meat 
off  a  bone. 

The  same  sort  of  development  on  flat  areas  separated  by  valleys 
lined  with  a  softer  epidermis  results  in  the  "scale"  of  the  reptiles.  The 
legs  of  birds  show  such  structures  well  developed,  and  in  the  entire  skin 
of  a  fowl  or  pigeon  early  rudiments  of  this  formation  maybe  seen.  Such 
scales  are  shed  periodically  and  replaced  by  new  ones  which  are  devel- 
oped beneath  them.  The  method  of  fission  between  the  old  and  new 
layers  is  unexplained.  It  can  be  well  seen  in  the  snake. 

It  is  the  analogous  structures  which  are  developed  from  invaginated 
regions  of  an  outer  stratified  epithelium  that  show  the  most  perfect  or- 
ganization, however.  Such  are  the  hair  and  feathers  of  the  mammals 
and  birds.  Of  these  two,  the  feather  seems  to  be  the  highest  specializa- 
tion. 

Hair  is  formed  in  general  as  follows.  —  It  begins  in  the  embryo 
mammal  as  a  thickening  of  the  epidermis,  particularly  of  the  basal  layer. 
This  thickening  soon  develops  into  an  invaginated  pocket  filled,  of  course, 
with  the  basal  cells.  This  pocket  deepens  into  a  long  tube  whose  fundus 
is  widened  and  filled  with  the  cells  of  a  considerably  thicker  epithelium 
than  that  which  lines  the  sides.  The  bottom  of  this  bulb  is  then  evagi- 
nated for  a  short  distance  by  the  growth  of  a  mesodermal  papilla,  and  the 
epithelium  on  this  papilla  begins  to  grow  much  more  rapidly  than  any 
of  the  rest,  forcing  a  pointed  mass  up  through  the  lumen  of  the  tube. 

This  mass  consists  of  a  central  core  which  is  the  hair  shaft  with  an 
outer  layer  called  the  inner  sheath.  This  sheath  travels  with  the  hair 


INTEGUMENT 


367 


shaft  and  slides  against  the  cells  of  the  epithelium  which  lines  the  hair 
tube.  These  latter  form  the  several  layers  of  the  outer  hair  sheath  which 
is  thus  a  modified  form  of  stratified  epithelium  continuous  with  that  of 
the  surface.  At  about  two  thirds  of  the  distance  to  the  surface  the  inner 
sheath  degenerates,  and  the  hair  is  surrounded  directly  by  the  epithelial 
layers  of  the  outer  sheath. 

This  epithelium,  at  two  places,  about  a  third  to  a  half  of  the  distance 
from  the  surface  to  the  bulb,  is  thickened  by  the  growth  of  its  basal  layer. 
The  upper  of  these  is  the  developing  sebaceous  gland  which  is  described 


ep.  3. 


< 

^o   ep.  3. 


FIG.  331. — Section  of  a  hair  root,  just  under  the  skin,  m.,  medulla;  i.ep.s.,  inner  epithelial 
sheath,  Henle's  layer  and  Huxley's  layer;  o.ep.s.,  outer  epithelial  sheath;  h.m.,  hyaline 
membrane;  /./.,  fibrous  layer. 

under  lubrication.  The  lower  is  a  center  for  the  renewal  of  the  hair, 
which  falls  out  after  its  term  of  usefulness  is  over  (Fig.  331). 

The  layers  of  the  outer  hair  sheath  are  interesting.  The  upper  is  a 
continuation  of  the  outer  epidermis.  The  stratum  corneum  extends 
down  as  far  as  the  sebaceous  gland,  where  its  place  is  taken  by  the  inner 
sheath.  The  stratum  granulosum  extends  farther,  and  only  the  basal 
layer  can  be  traced  continuously  to  the  papillae. 

In  the  birds  an  epithelial  structure  is  produced,  somewhat  similar  to 
hair,  and  called  the  feather.  Like  the  hair,  the  feather  begins  in  the 
embryo  as  a  thickening  of  the  very  thin  epithelium  which  covers  the 
integument  (Fig.  332).  The  same  layers  are  found  here  as  were  seen 
where  the  hair  developed  in  the  mammal,  with  the  small  differences  of 
detail,  that  in  the  hair  anlage  a  thicker  layer  of  epithelium  is  to  be 


368 


HISTOLOGY 


found,  and  not  so  pronounced  a  thickening  of  the  mesodermal  rudiment 
of  the  papillae  as  is  to  be  seen  in  the  feather  anlage. 

The  feather  is  very  complicated  in  its  development,  and  we  shall 

describe     the     com- 
pleted structure  first 
(Fig-  333)-     It  con- 
-     — ^^     sists  of  a  more    or 
^,»9-^r&°-^  '^o*0^*  ' 

^sgfea&fe 


FIG.  332. — Very  early  rudiment  of  a  down  feather  in  a  pigeon. 
p.,  papilla;  ep.,  epidermis.     (After  DAVIES.) 


less  long  quill  which 
is  set  in  a  follicle 
much  as  a  hair  is. 
The  quill  is  a  hollow 
tube  in  which  is  usu- 
ally found  a  loose  parchment-like  series  of  irregular  lamellae  which 
divide  it  into  several  chambers.  At  its  proximal  extremity,  in  the  bot- 
tom of  the  follicle,  it  opens  through  a  slightly  constricted  neck,  and  a 
mass  of  vascular  mesodermal  pulp,  covered  with  the  stratified  epithe- 
lium of  the  follicle,  reaches  up  for  a  distance  into  its  lumen.  This 
.structure  is  the  feather  papilla,  and  the 
opening  is  known  as  the  interior  umbilicus. 
The  quill  rests  against  the  sides  of  the  fol- 
licle which  are  covered  with  the  invagi- 
nated  stratified  epithelium.  Distally,  the 
short  quill  is  extended  into  a  longer  and 
somewhat  smaller  shaft  known  as  the  rachis. 
At  the  point  of  juncture  there  is  another 
small  opening,  the  superior  umbilicus. 

The  rachis  has  a  solid  wall  with  an  alve- 
olar core,  and  from  each  of  its  two  sides 
springs  a  longitudinal  row  of  more  closely 
set,  parallel,  thin  plates  called  the  barbs. 
Projecting  again  from  the  upper  or  distal 
edge  of  each  barb  are  two  rows  of  very 
small  processes  known  as  the  barbules. 
The  anterior  row  of  barbules  is  provided  on 
its  lower  side  with  a  series  of  tiny  hooks, 
while  the  posterior  row  is  shaped  into  a 
series  of  tiny,  ragged-edged  plates  so  placed 
that  the  anterior  barbules  of  the  next  barb 
will  catch  in  them  and  hold  as  strongly  as 
their  elasticity  will  permit.  When  forced 
apart,  the  hooks,  if  uninjured,  will  catch 
again  the  next  time  they  touch  the  plates  of  the  neighboring  barbule. 
All  these  structures,  the  quill,  rachis,  barbs,  and  barbules  are  dead 


FIG.  333.  —  Lower  portion  of  a 
nearly  completed  down  feather  in 
the  pigeon,  q.,  quill;  cy.c.,  cyl- 
inder cell  layer;  d.f.,  beginning 
of  the  growth  of  the  definitive 
feather.  (After  DAVIES.) 


INTEGUMENT 


369 


cellular  matter  derived  from  the  epidermis  of  the  feather  papilla.     When 
the  feather  is  pulled  out  or  is  molted,  the  papilla  is  left  behind,  and 


FIG.  334.  —  A  second  stage  in  the  growth  of  a  down  feather,  ep.l.,  epitrichial  layer  of  epider- 
mis; int.,  intermediate  layer  of  epidermis;  cy.l.t  basal  layer  of  cylinder  cells  of  epidermis; 
bl.v.,  blood  vessel  in  papilla.  (After  DAVIES.) 

begins  to  form  a  new  feather  by  the  growth  from  its  proximal  dorsal 
surface  of  a  new  papilla. 

The  first  feather  to  be  formed  by  the  embryonic  papilla  such  as  was 
mentioned  on  the  preceding  page  is  called  the  down  feather.  The  em- 
bryonic papilla  (Fig.  334)  elongates,  the  while  that  it  settles  into  the 
skin.  The  whole  structure  now  consists  of  an  evaginated  region  of  the 

bottom  of  an  invaginated 
region.  The  outermost  epi- 
dermal layer,  however,  does 
not  dip  into  the  fundus  but 
reaches  from  the  surface 
directly  up  and  over  the 
feather  rudiment. 

The  epidermis  is  very 
thick  on  the  papilla,  and 
soon  begins  to  show  a  dif- 
ferentiation. The  basal 
layer  is  thrown  into  longi- 
tudinal folds  whose  inner 
flexures  are  round,  and 
whose  outer  folds  are 
sharp  and  plate-like  (see 
Fig.  335).  The  mesodermal 
tissue  of  the  papilla  extends 
as  a  thin  layer  out  into  the 
plates.  The  middle  and  basal  layers  of  the  epidermis  are  now  divided 
into  a  series  of  cylinders  running  lengthwise  on  the  papilla. 

2B 


FlG.  335.  — Transverse  section  through  a  somewhat  older 
pin  feather  than  that  represented  in  Fig.  334.  bl.v., 
blood  vessels  in  the  papilla;  ep.l.,  epitrichial  layer  of 
epidermis;  int.,  intermediate  layer  of  epidermis;  cy.l., 
basal  layer  of  cylinder  cells.  The  longitudinal  cylin- 
ders are  partly  of  cylinder  cells.  The  longitudinal 
cylinders  are  partly  formed  on  the  dorsal  side  of  this 
young  structure.  (After  DAVIES.) 


370 


HISTOLOGY 


if i       ^Z^^ 

\X/^*n*».S35 


The  two  or  three  outer  layers  of  the  epidermis  not  involved  in  this 
cylinder  formation  now  form  a  stratified  and  partly  cornified  layer,  the 

sheath  layer,  on  the  outside  of 
the  rudimentary  feather  (Fig. 
336).  The  outer  edges  of  the 
plates,  where  they  touch  this 
sheath,  become  flattened  and 
broadened  to  partly  cut  the 
cylinders  off  from  the  sheath 
layer.  This  is  more  apparent 
distally  and  disappears  proxi- 
mally. 

Finally    the    cylinder    cells 
form  concentric  layers  in  each 
FIG.  336.— A  later  stage,  than  Fig.  335,  in  the  de-    cylinder,    and   cornify   in   the 

velopment  of  a  down  feather  of  the  pigeon.     The      j:_t-i      _.._<.      /F-          ,,,_\       rpi 

cylinders  are  well  defined,  but  have  not  yet  cut  off      '  l     Pari     vrig-     337 )• 

their  inclosed  intermediate  cells,  which  now  form      sheath    falls    off    and    the    pulp 


the  longitudinal  plates,  from  the  epitrichial  layer. 
Letters  same  as  in  last  figure.     (After  DAVIES.) 


tissue     withdraws     its     blood 
supply    and    dries    up.    This 

leaves  the  cylinders  free,  but  still  connected  at  their  basal  ends  with 

the  hardened  base  of  the  papilla  which  has  not  formed  this  part  of  its 

epidermis  into  cylinders.     We  thus  have,  as  a  completed  structure,  a 

quill-like  base  from  whose  distal 

end    arises   a   circular  row  of      M.  v. 

filamentous     processes     called 

the  barbules. 

This  down  feather  is   soon 

lost,  usually  by  being  pushed 

out  by  the  new  or  permanent 

feather  which  takes  its  place. 

This  second  feather,  which  like 

all  its  successors  is  known  as  a 

definitive  feather,    arises    as    a 

new  papilla  that  grows  out  of 

the  base  of  the  old  one  which  it 

reab  sorbs.     Like  the  first  pa- 
pilla, it  is  a  dermal  pulp,  covered 

with  a  thick  epidermis.    This 

epidermis,  when  the  papilla  is 

large    enough,    begins    also    to 

develop  a  series  of  longitudinal 

folds  much  like  those  of  the  down  feather  rudiment.     Several  of  the 

folds  on  the  dorsal  side  of  the  papilla  continue  straight  and  become 


cy.l. 


FIG.  337. — Transverse  section  of  a  young  down 
feather  near  maturity,  int.,  longitudinal  plates 
of  what  were  formerly  the  intermediate  cells; 
cy.l.,  cylinder  cells  which  now  surround  the  lon- 
gitudinal plates.  Other  lettering  same  as  in  pre- 
ceding figure.  (After  DAVIES.) 


INTEGUMENT 


37* 


fused  together  as  one.  They  grow  extensively  and  become  the  rachis. 
Figure  338  shows  a  cross  section  of  a  feather  that  shows  this  devel- 
opment. 

The  outer  cylinders  become  arranged  obliquely  on  the  papilla  pulp 
so  that  they  form  two  series  each  attached  in  a  line,  one  to  one  side,  the 
other  to  the  other  side  of  the  rachis.  They  now  lie  obliquely  on  the  pa- 
pilla, and  as  the  rachis  emerges  from  the  skin  it  drags  them  in  two  rows 
with  it.  They  continue  soft  and  developing  on  the  bottom  until  the 
head  of  the  quill  emerges  with  the  last  of  them,  thin  and  poorly  developed, 
as  a  ring  of  downy  barbs  which  surround  the  superior  umbilicus. 


FIG.  338.  —  Transverse  section  of  a  large  vane  feather  or  permanent  feather  in  an  early  stage 
of  development,  v.,  point  at  which  two  of  the  longitudinal  plates  are  united  and  enlarged 
to  form  the  vane  or  rachis.  The  remaining  plates  become  specialized  to  develop  the  barbs 
and  barbules  of  the  feather.  (After  DAVIES.) 


The  growth  of  the  individual  cylinder  into  a  barb  of  the  definitive 
feather  is  complicated  by  the  fact  that  the  latter  develops,  in  the  course 
of  its  growth,  the  two  series  of  barbules  on  its  sides.  It  is  the  inner  rod- 
like  region  of  the  cylinder  which  hardens  into  the  barb,  while;- its  outer 
cells  become  arranged  in  slanting  rows  and  harden  into  the  barbules, 
forming  the  hooks  in  one  set  and  the  plates  in  the  other.  Space  forbids 
us  to  go  further  into  the  details  of  this  complicated  development  which 
has  been  worked  out  by  H.  R.  Davies  and  others.  Figure  339  shows  a 
transverse  section  of  a  single  barb  in  process  of  development. 

Scales  of  Fishes.  —  Our  last  example  of  an  integumental  structure 
used  for  mechanical  protection  is  peculiar,  in  that  it  is  developed  in  the 


372 


HISTOLOGY 


cy.l. 


h. 


FIG.  339.  —  Parts  of  two  longitudi- 
nal plates  from  a  permanent 
feather  somewhat  more  advanced 
in  development  than  in  the  pre- 
ceding figure.  One  of  these 
plates  of  cells  will  produce  a 
single  barb  with  its  double  row 
of  barbules.  cy.l.,  cylinder  layer 
of  cells;  b.,  cells  which  will  form 
the  barbules;  h.,  horny  covering. 
(After  DAVIES.) 


mesodermal  element  of  the  skin  below  the  epithelium  which  takes  no 
part  in  its  formation.  This  structure  is  the 
scale  found  on  the  body  surface  of  the  tele- 
ost  fish. 

The  skin  of  a  fish  consists  of  a  thick 
layer  of  connective  tissue  laid  down  in  thin 
horizontal  layers  and  forming  the  base  on 
which  rests  a  heavy,  stratified  epithelium. 
In  the  embryo  these  layers  are  distinctly 
separated  from  one  another  by  a  clear, 
sharp-cut  line,  the  basement  membrane. 
The  outermost  layer  of  the  connective  tis- 
sue, or  cutis,  changes  from  the  flat,  highly 
specialized  connective-tissue  cells  to  rounded 
mesodermal  cells  with  oval  nuclei  whose 
chromatin  is  arranged  in  a  pattern  that  re- 
sembles the  arrangement  in  the  nuclei  or 
epithelial  cells.  This  layer  slowly  increases 
by  mitotic  divisions.  Later  in  the  develop- 
ment (trout  of  forty-five  millimeters),  it  can 
be  seen  that  this  layer  has  become  divided  into  two  layers  by  a  deposit 
which  the  cells  are 
laying  down  be- 
tween them  (Fig. 
340).  This  layer  of 
homogeneous  and 
dense  material  rep- 
resents a  longitudi- 
nal section  of  a  thin 
oval  plate  called  the 
scale.  It  increases 
in  size  (which  is 
represented  by 
length  in  our  draw- 
ing) and  also  in 
thickness. 

These  scales  oc- 
cur, placed  in  a  reg- 
ular pattern,  over 
most  of  the  fish's  in- 
tegument. At  first 
they  do  not  interfere  with  one  another,  but  later  they  increase  so  in 
size,  especially  on  the  posterior  edge,  that  they  overlap,  and  come  to  lie 


conn.  t. 


FIG.  340.  —  Several  basal  epithelial  cells  (b.c.)  resting  on  the  rudi- 
ment of  a  young  scale  of  an  embryo  trout,  conn.t.,  unmodified 
connective  tissue  of  the  corium;  /.,  the  two  distal  layers  of  con- 
nective tissue  constituting  the  scale  follicle.  The  young  scale, 
in  section,  is  indicated  by  a  dotted  line.  (After  NUSBAUM.) 


INTEGUMENT 


373 


as  a  series  of  alternate  and  overlapping  plates,  like  the  shingles  on  a 
house.  They  push  the  posterior  part  of  the  connective-tissue  pocket  in 
which  they  lie,  up  into  the  thick  epithelium  from  which  they  are  sepa- 
rated by  but  a  thin  layer  of  connective  tissue  and  their  own  matrix 
cells. 

These  matrix  cells  of  the  papilla,  which  were  at  first  round  and  plump, 
now  become  drawn  out  into  a  very  thin  epithelial-like  layer.  Those  on 
the  upper  side  of  the  scale  lay  down  as  a  rule  a  more  uneven  surface  on 
the  scale  than  those  in  the  lower  layer.  In  many  fishes  the  upper  sur- 
face, especially  on  its  posterior  part,  is  covered  with  many  points,  knobs, 
or  other  processes  which  cause  it  to  be  called  a  ctenoid  scale.  During  its 


•-^••*W;-:;>. 


4& 


S^^^^Wiliii?-^-  ; 


FIG.  341. — Two  scales  near  maturity  in  the  skin  of  a  young  trout,  b.m.,  basement  membrane 
of  the  overlying  striated  epithelium;  conn.t.,  connective-tissue  corium;  /.,  scale  follicle 
formed  from  connective  tissue  of  corium.  The  mesodermal  formative  cells  lie  on  the  distal 
and  proximal  surfaces  of  the  scales,  forming  a  very  thin  layer  on  each  surface.  (After 

NUSBAUM.) 

earliest  stages  the  young  scale  lies  almost  flat  or  parallel  with  the  body 
surface.  As  it  grows  in  extent,  the  posterior  end  is  tilted  up.  In  Figure 
341  the  shrinkage  of  the  layers  has  caused  the  scales  to  stand  up  much 
more  than  they  do  in  life.  The  ganiod  and  selachian  fishes  also  have 
mesodermal  structures  developed  in  the  skin  for  protection. 

The  integument  of  the  echinoderms  also  shows  a  set  of  hard  plates 
consisting  of  a  deposit  of  lime  in  its  tissues.  This  series  is  another  ex- 
ample of  the  comparatively  rare  cases  in  which  these  mechanical  pro- 
tective structures  are  developed  in  the  cutis. 

The  epidermis  in  these  animals  is  a  thin,  simple  epithelium,  and  not 
fitted  to  stand  either  abrasion  or  pressure.  The  underlying  plates  of  the 
cutis  not  only  protect  the  body  from  pressure  by  their  rigidity  but  they 
also  protect  the  epidermis  from  abrasion  by  the  formation  of  outlying 


374  HISTOLOGY 

points  which  loose  their  own  epithelial  covering,  but  succeed  in  preserv- 
ing that  of  the  wider  surfaces  which  lie  between  them. 

The  shape  of  these  plates  varies  exceedingly.  Some  are  flat  and 
hard.  Others  are  spongy  in  texture,  while  in  the  Holothuria  they  form 
isolated  plates  of  various  patterns,  some  of  them  very  beautiful,  as  the 
anchor  plates  in  some  of  the  Synaptas. 

Technic.  — The  technic  of  this  group  of  tissues  varies  from  the  sim- 
plest and  easiest  to  the  most  difficult,  mechanically,  owing  to  the  great 
variety  in  which  the  protective  substances  and  cells  are  developed.  The 
difficulty  lies  in  the  hardness  of  these  parts,  and  the  consequent  breaking, 
irregularity,  and  unmanageability  of  the  sections.  The  heterogeneous 
tissues  are  the  worst  in  this  respect.  The  remedy  in  most  cases  is  an 
extra  sharp  and  good  knife,  and  a  deliberate  and  careful  handling  of 
the  sections  when  they  have  been  secured.  In  case  the  hardness  is  due 
to  some  salt  of  lime,  the  tissue  should  be  decalcified  either  by  the  fixative 
or  by  a  subsequent  treatment  with  hydrochloric  acid  and  phloroglucin. 
When  it  is  due  to  chitin  and  connective-tissue  substances  the  problem 
is  not  so  easy.  Most  methods  of  softening  these  materials  injure  the 
structure  and  staining  power.  It  is  therefore  better  to  use  such  processes 
only  as  a  last  resort  and  to  first  try  to  get  sections  with  a  very  good  knife 
by  the  ordinary  way.  Sometimes  it  is  best  to  saw  off  thick  sections  and 
then  to  grind  them  down  on  a  stone  in  oil.  If  soft  parts  are  associated 
with  the  hard  parts  and  must  be  preserved,  the  whole  mass  must  be 
fixed  and  then  embedded  in  rosin,  in  which  condition  the  entire  structure, 
soft  and  hard  alike,  may  be  sawed  off  and  ground  and  then  mounted  after 
the  rosin  has  been  dissolved.  In  some  instances  teasing  of  fresh  or  fixed 
material  and  hand  sections  of  fresh  material  will  give  good  results  after 
all  other  methods  have  failed. 

LITERATURE 

But  few  papers  deal  with  the  integument  as  a  whole.  Accounts  of  this  organ  usually 
form  parts  of  more  general  descriptions  or  articles  based  upon  researches  that  have  been 
made  on  some  particular  component  tissue  of  the  integument.  Papers  and  descriptions 
may  be  found  in  the  following  places. 

SCHNEIDER,  K.  C.     "Lehrbuch  der  vergleichenden  Histologie,"  Jena,  1902. 
HALLER,  B.     "Lehrbuch  der  vergleichenden  Anatomic,"  Jena,  1904. 

Read  parts  of  Parker  and  Haswell,  Lang,  and  other  Zoologies.     Accounts  of  the  skin 
in  medical  histologies. 
BLASCHKO,  A.     "Beitrage  zur  Anatomie  der  Oberhaut,"  Arch.f.  mik.  Anat.,  Band  XXX, 

S.  498,  1887. 
CERFONTAINE,  P.     "  Recherches  sur  le  Systeme  cutane"  et  sur  le  Systeme  musculaire  du 

Lombrie  terrestre,"  Arch.  Biol.,  Band  X,  1890. 
TOLDT,  C.      "Uber  den  feineren  Bau  der  Cuticula  von  Ascaris  megalocephala,"  Arb.  Z. 

Inst.  Wein,  Band  XI,  1899. 
BIEDERMANN,  W.     "  Untersuchungen  iiber  Bau  und  Entstehung  der  Molluskenschalen," 

Jen.  Zeits.  naturwiss.,  Band  XXXVI,  1901. 


MECHANICAL  PROTECTION  AND  POISONS 


375 


DAVIES,  H.  R.     "  Die  Entwicklung  der  Feder  und  ihre  Beziehung  zu  anderen  Integumen- 

tatgebilden,"  Morph.  Jahrbuch.,  Band  XV. 
NUSBAUM,   JOSEF.     "Zur  Histogenese  der  Lederhaut  und  der  Cycloid  Schuppen  der 

Knochenfische,"  Anat.  Anz.,  Band  XXX,  Nos.  11—12,  1907. 
VITZON,  ALEX-NICH.     "  Recherches  sur  la  struct,  et  format,  des  integuments  chez  les  Crus- 

tecis  Decapodes,"  Arch,  de  Zool.  Exp.  et  Gen.,  t.  x.,  pp.  451-576,  pis.  XXIII-XXVIII, 

Paris,  1882. 
HERRICK,  F.  H.     "The  American  Lobster,"  U.S.F.  Comm.  Bull,  for  1895,  pp.  1-252, 

plates  A-J  and  1-54. 


OFFENSIVE  MECHANICAL  PROTECTION  AND   POISONOUS  FLUIDS 

Offensive  mechanical  protection  is  a  function  of  many  kinds  of 
epithelia  and  the  products  of  these  epithelia  as  well  as  dermal  structures 
associated  with  them.  It  is  but  remotely 
removed,  so  far  as  the  structure  of  the  tis- 
sues that  perform  it  is  concerned,  from  the 
passive,  mechanical  protection  treated  of 
in  the  last  section.  A  series  of  merely 
ornamental  points  on  an  insect  larva 
might  be  developed  by  selection  or  other- 
wise into  stinging  organs  and  spines.  Also 
the  same  sort  of  development  of  the  har- 
dened outer  layer  of  the  stratified  epithe- 
lium that  results  in  a  downy  hair  or 
feather  is  sometimes  used  to  produce 
claws  and  horns.  The  principal  factor 
that  leads  the  writers  to  separate  these 
mechanically  offensive  organs  from  the 
simple  protective  structures,  histologically, 
is  the  fact  that  many  of  them  are  associ- 
ated with  poison  glands.  The  associated 
poison  glands  will  also  be  described  at  the 
same  time. 

We  shall  first  consider  three  intracel- 


FIG.  342. — Part  of  a  vertical  section 
of  a  triclad  worm.  The  outer  layer 
consists  of  a  simple  layer  of  colum- 
nar, epithelial  cells  containing  tri- 
chocysts  in  their  distal  cytoplasm 
and  showing  a  modified  and  pro- 
tective border.  Beneath  this  the 
underlying  tissues  have  become  ar- 
ranged in  layers  with  reference  to 
the  body  surface.  (After  PARKER 
and  HAS  WELL.) 


lular  forms;  the  trichocysts  of  Infusoria  (see  Fig.  245);  the  rhabdiles  of 
turbellarian  worms,  and  the  nettle-cells  of  ccelenterates.  Secondly,  the 
extracellular  forms  from  columnar  epithelia  and  the  multicellular  forms 
found  developed  from  stratified  epithelia. 

The  simplest  forms  of  such  organs  are  the  rod-like  trichocysts  of 
Paramcecium  and  other  Infusoria,  and  the  so-called  rhabdites  and  stylets 
found  in  the  turbellarian  and  nemertean  worms  (Fig.  342).  These  are 
short  rods  pointed  at  the  outer  end  and  formed  in  the  cytoplasm,  usually 
of  the  cells  that  contain  and  use  them.  They  may  be  formed  by  internal 


376 


HISTOLOGY 


cells  in  the  higher  forms,  however,  and  passed  from  cell  to  cell  until 
delivered  for  use  at  the  surface.     It  is  probably  an  organ  of  defense,  and 

is  usually  surrounded  by  a  layer 
of  slime  that  may  contain  poison. 
The  rhabdites  are  developed  in 
some  cases  into  large  structures 
called  stylets  and,  by  some,  these 
are  thought  to  be  a  transition 
from  the  rhabdite  into  the  next 
form  of  offensive  mechanical  pro- 
tection that  we  shall  study,  the 
nematocyst  or  nettle  cell  of  the 
coelenterates. 

The  nettle  cell  or  cmdoUast  is 
an  organ  that  is  developed  in  the 
cytoplasm  of  a  single  cell,  and  is  a 
marked  example  of  a  regular  and 
complicated  structure  of  great 
efficiency  and  delicate  adjustment 
formed  by  an  apparently  amor- 
phous mass  of  protoplasm.  It 
much  resembles  the  trichocyst  of 
some  Infusoria  with  which  it  is 
probably  homologous  (see  Fig. 

245)- 

The  cnidoblast  or  nettle  cell  is 
found  in  the  basal  layer  of  the 
epidermis  in  Hydra,  and  when 

FIG.  343-—  A,  young  but  fully  formed  cnidoblast      young    it    bears    no    trace    of    its 

future     development     and     func_ 

tion.  When  called  upon  to  de- 
velop nematocysts,  or  stinging 
sacs,  by  the  need  of  them  on  the  surface,  it  produces  them  in  its 
distal  cytoplasm  which  is  enlarged  and  drawn  somewhat  toward  the 
surface  at  this  time. 

The  nematocyst  first  appears  as  a  small,  rounded  mass  of  greater 
density  than  the  surrounding  cytoplasm  and,  as  it  enlarges,  it  develops 
a  less  dense  interior.  Its  distal  wall  is  invaginated  into  this  interior  as 
a  hollow  thread  which,  when  developed,  lies  coiled  in  the  space.  The 
external  opening  of  the  lumen  of  this  invaginated  tube  thread  is  closed  by 
a  thin  cover.  The  interior  of  the  sac-like  portion  or  capsule  is  filled  with 
the  secretion  which  is  a  fluid  in  the  ripe  organ. 

The  mature  capsule  (Fig.  343)  with  its  contained  parts  does  not  lie 


of  Hydra;  B,  freshly  discharged  cnidoblast  of 
same  animal;  t.,  thread;  nem.,  nematocyst; 
CMC.,  cnidocil;  nu.,  nucleus  of  cnidoblast. 
(After  SCHNEIDER.) 


MECHANICAL  PROTECTION  AND  POISONS  377 

primarily  in  the  cytoplasm,  but  in  a  vacuole  space  which  is  bounded  by 
another  distinct  membrane,  the  vacuole  membrane.  The  walls  of  this 
membrane  are  continued  from  its  sides  down  to  the  lower  part  of  the  cell 
by  a  separate  wall  of  membrane.  Between  the  capsule  and  the  vacuole 
membrane  lies  a  very  small  amount  of  fluid  in  the  mature  cnidoblast. 

One  other  important  organ  is  developed  in  the  cytoplasm  in  connec- 
tion with  the  nematocyst.  This  is  a  pointed,  chitinous  rod  called  the 
cnidocil.  This  rod  projects  distally  beyond  the  surface  of  the  surround- 
ing epithelium,  and,  when  touched,  it  acts  so  as  to  stimulate  the  cell,  con- 
tract the  capsule,  and  force  the  thread  to  evert.  This  eversion  is  a  very 
efficient  method  of  causing  the  thread  to  penetrate  the  body  surface  of 
an  enemy.  Spines  are  placed  on  the  inside  of  the  folded  or  invaginated 
thread  so  that  when  it  is  everted  they  will  come  out,  point  forward,  and 
penetrate  a  hard  body  first,  thus  making  a  way  for  the  softer  thread  to 
follow.  Thus,  the  method  of  eversion  does  not  involve  any  traction 
or  friction,  which  the  thread  is  too  delicate  to  bear.  Once  in  the  victim's 
body,  the  spines  are  thrown  backward  and  serve  to  retain  the  thread. 
In  passing  into  the  victim  the  thread  carries  a  poison  with  it. 

A  very  different  type  of  integumental  organ  of  offense  is  found  in 
some  of  the  Echmoderms,  the  sea-urchins  or  Echinoidea.  In  these  ani- 
mals the  body  is  covered  with  spines  which  project  from  the  surface  and 
are  movable  on  a  knob-like  process  of  one  of  the  plates  with  which  this 
animal's  skin  is  provided.  In  some  forms  the  spines  are  comparatively 
harmless,  but  the  spines  of  a  very  common,  black  urchin,  Diadema, 
found  on  all  tropical  coasts,  are  of  tremendous  length  and  exceedingly 
sharp. 

Such  a  spine  consists  of  a  core  of  mesodermal  origin  and  an  outer 
integumental  layer.  This  outer  layer  originally  has  an  epithelium  con- 
tinuous with  the  rest  of  the  body,  but  when  the  spine  is  mature  this  epithe- 
lium is  rubbed  off,  leaving  the  hard,  cortical  layer  of  lime  for  an  outside 
covering.  Near  the  base,  the  epithelium  persists  and,  in  Diadema,  is 
subject  to  interesting  modifications  (see  Chapter  XIII,  on  Eyes). 

The  internal  part  of  a  spine  of  Diadema  (Fig.  344)  consists  of  a  long, 
central  canal  for  circulation  surrounded  by  a  mesodermal  tissue  covered 
with  epidermis.  The  tissue  about  the  canal  is  built  out  into^a  number 
of  radial  plates  which  are  attached  longitudinally  to  the  canal-bearing 
core.  These  plates  meet  folds  of  the  integument  which  project  inward 
and  thus  form  a  series  of  tubular  spaces  in  which  the  lime  tissue  of  the 
spine  is  laid  down.  At  first  this  lime  is  an  almost  solid  rod,  but  as  the 
spine  grows,  a  connective-tissue  reticulum  is  formed  and  acts  as  a  basis 
for  the  additions  of  lime  as  shown  in  Figure  344  at  cal. 

The  base  of  the  spine  is  flexible,  owing  to  a  disk-shaped  "region  in 
which  no  lime  is  deposited,  and  where  the  connective  tissue  is  developed 


378 


HISTOLOGY 


centrally  into  a  ligament.  The  peripheral  mesodermic  tissue  of  this 
same  region  forms  a  circular  sheet  of  longitudinal  muscle  fibers  which, 
by  pulling  on  one  side  or  on  the  other,  is  able  to  move  the  spine  in  any 
desired  direction.  Around  the  base  of  most  echinoderm  spines  is  a 
ring  of  dermal  tissue  in  which  nerve  cells  and  their  fibers  cause  a  con- 
siderable thickening.  This  is  cut  tangentially  in  the  figure. 


mes.     pi. 


FIG.  344. — Section  through  the  base  of  a  poison  spine  of  the  echinoderm,  Diadema  setosum. 
mes.pl.,  mesodermal  plates  radiating  from  the  central  canal  (here  occupied  by  one  of  the 
amoeboid  blood  cells);  x,  spaces  occupied  by  the  youngest  plates  of  lime;  cat.,  organic  base 
of  the  older  calcareous  tissue;  int.,  outer  integument;  mus.f.,  some  of  the  muscle  fibers 
which  move  the  spine;  n.in.,  integument  containing  the  mass  of  nerve  tissue.  X  520. 


The  spine  makes  a  painful  wound,  and  its  outer  tissues  probably 
secrete  an  irritating  fluid.  In  another  form,  Msthenosoma,  the  tip  of  the 
spine  is  enormously  enlarged  and  its  upper  point  invaginated  into  a 
pocket.  From  the  base  of  this  pocket  arises  a  spicule  which  is  a  continua- 
tion of  the  core  of  the  spine  and  the  whole  sac  is  filled  with  a  poisonous 
fluid  secreted  by  the  lining  cells  (Fig.  345).  When  the  delicate  head  of 
the  spine  Is  touched  by  any  creature,  the  top  breaks  or  caves  in,  and  the 
spicule  is  forced  into  the  victim's  flesh.  At  the  same  time  the  muscles 


MECHANICAL  PROTECTION  AND  POISONS 


379 


of  the  enlarged  head  force  the  poison  out  into  the  wounds,  causing  much 
pain  in  a  man,  and  death  to  some  smaller  animals. 

The  insects  have  developed  formidable  weap- 
ons by  which  they  kill  their  prey  and  injure  their 
larger  enemies.  The  best  known  examples  are  the 
bees  and  mosquitoes.  The  apparatus  in  the  butcher 
bee  (or  ground  hornet),  Scolia  dubia,  consists  of  a 
cuticular  formation,  the  sting,  which  is  made  up  of 
several  parts  and  worked  by  special  muscles  and 
nerves,  and  a  poison  gland  with  its  reservoir,  which 
is  an  invaginated  portion  of  an  internal  integu- 
ment, if  the  lower  part  of  the  intestine  can  be  so 
called.  As  the  poison  gland  is,  perhaps,  the  more 
important  from  our  point  of  view,  we  shall  devote 
our  time  to  that  tissue,  merely  remarking  that  the 
several  parts  of  the  sting  and  its  sheath  are  elon- 
gated processes  of  chitin  formed  by  long,  hypoder- 
mal  cells. 

The  duct  and  reservoir  are  lined  by  a  very 
delicate  hypodermis,  covered  by  a  thin  and  flexible 
chitin.  The  real  poison  gland  is  a  tubular  invagi- 
nation  lined  with  a  cuticle  whose  walls  are  invagi- 
nated into  many  fine  tubes.  These  tubes  extend 
proximally  into  a  series  of  elongated  columnar 
gland-cells  in  whose  cytoplasm  they  pursue  a  short 
course  and  end  with  a  cylindrical  enlargement  which 
appears  to  act  as  a  physiological  filter  (Fig.  346). 
The  poisonous  secretion  is  conducted  by  these  tubes 
to  the  sting,  from  which  it  runs  in  fine  streams 
that  emerge  behind  the  barbs.  A  layer  of  thin 
cells  with  very  small  nuclei  extends  between  the  gland-cells  and  the 
cuticle.  Their  function  and  origin  must  be  the  same  as  in  the  odorous 
gland  of  Belostoma,  which  see. 

Among  some  insects,  particularly  the  larvae  of  Lepidoptera  are  found 
some  poisonous  spines  which  are  very  efficient  in  their  structure  and 
operation.  They  are  specializations  of  the  insect  hair,  which  can  be 
modified  to  form  structures  for  so  many  other  purposes. 

In  the  case  that  we  shall  select  for  examination,  Sibine  stimulea,  the 
hairs  are  compound  structures,  extending  from  several  wart-like  pro- 
jections on  the  sides  of  the  body,  and  particularly  from  four  large  horn- 
like processes,  two  of  which  arise  on  the  head  and  two  on  the  tail  of  the 
larva.  The  particular  hair  that  we  are  examining  is  represented  by  Figure 
347,  which  shows  an  oblique  section  of  the  basal  part  at  the  point  where 


FIG.  345-— Tip  of  the 
spine  of  the  sea-urchin 
jEsthenosoma.  p.s., 
poison  sac;  /.,  lance  to 
puncture  enemy;  mus., 
muscular  cushion  to 
squeeze  out  poison;  c., 
epithelial  cap  which  is 
ruptured  when  the 
spine  is  used.  (After 
P.  and  F.  SARASIN.) 


38o 


HISTOLOGY 


it  emerges  from  the  thick  cuticle  of  the  horn-like  process.  This  cuticle 
on  the  horn  is  very  thick  indeed  and  is  lined  internally  by  a  well-developed 
hypodermis,  inside  of  which  is  some  connective  tissue  and  a  few  blood 
cells  and  other  cells.  Where  a  poison  hair  takes  its  origin  the  hypoder- 
mis is  evaginated  to  form  a  cylindrical  layer  extending  obliquely  up 
through  the  cuticle  and  finally  emerging  at  the  surface  as  a  hair-like 
structure  covered  with  its  own  thinner  cuticle  which  is,  of  course,  con- 
tinuous with  the  thick  layer.  This  whole  structure  is  extended  within 
the  entire  length  of  the  hair,  which  is  closed  on  the  end  except  for  a 
number  of  fine  openings  through  which  the  poison  can  pass  out. 


FIG.  346.  — Three  poison  cells  from  the  epithelium  lining  the  poison  gland  of  the  ground  hornet, 
Scolia  dubia.  p.n.,  poison  cell  nuclei;  CM.,  cuticle  on  distal  surface;  hyp.n.,  nuclei  of  the 
hypodermal  cells  that  form  the  cuticle.  At  d.  is  the  duct-like  invagination  of  the  cuticle 
which  ends  in  the  poison  sieve  or  secretion  sieve  at  s.  Parts  of  others  may  be  seen  in  the 
other  cells.  X  1200. 


The  poison  is  produced  by  a  large  cell  lying  inside  the  hair  and  its 
hypodermis,  at  about  the  level  of  the  hair's  emergence  from  the  horn. 
It  is  a  very  large  and  thick  cell,  with  a  large  branched  nucleus,  and  it 
often  looks  to  be,  and  may  be,  a  syncytium.  Its  large  cytoplasmic  body 
is  hollow  at  its  distal  part,  and  this  hollow  end  is  produced  into  a  tube 
which  carries  the  poison  up  into  the  hair  inside  of  the  hypodermis. 

The  presence  of  openings  in  the  hair  to  permit  the  poison  to  escape 
has  been  questioned  or  even  denied  by  some  writers.  The  present  writers 
have  not  been  able  to  see  them  and,  therefore,  think  that  the  great  irri- 
tation (which  has  certainly  been  felt)  is  due  to  the  breaking  off  of  the 
spines  in  the  flesh,  and  the  consequent  introduction  of  the  poison.  No 


MECHANICAL  PROTECTION  AND  POISONS 


381 


muscular  apparatus  has  been  found  that  could  be  used  to  inject  the 
venom. 

The  Arachnids  show  a  rather  remarkable  histological  advance  on  the 
usual  insect  types  in  regard  to  their  poison  glands.     In  place  of  the  very 


FIG.  347.  —  Slightly  oblique  section  through  the  base  of  a  poison  hair  of  Sibine  slimulea  where  it 
leaves  the  skin,  p.c.,  poison  cell  (a  syncytium)  with  a  hollow  lumen  to  carry  the  poison 
outward,  hyp.c.,  hypodermal  cells  which  make  the  hair  cuticle.  X  435. 

remarkable  secreting  cells,  with  the  cuticle  that  lines  the  usual  form  of 
insect  poison  gland,  we  find  a  gland  whose  secreting  cells  are  naked  dis- 
tally  and  which  possess  no  peculiar  intracellular  differentiations. 

The  poison  glands  of  the  spider,  Lycosa  sp.,  form  a  good  example. 
These  glands  are  integumental  invaginations  from  the  tips  of  the  biting 
mandibles,  and  they  extend  into  the  anterior  part  of  the  thorax. 


382 


HISTOLOGY 


basement     membrane 
is    homogeneous    and 


A  transverse  section  of  this  tubular  gland  shows  the  following  main 
layers.     A  single  outer  layer  of  large  muscle  fibers  which  extend  the 

length  of  the  gland  and  are 
consequently  seen  in  transec- 
tion.  The  sections  are  roughly 
square  or  triangular  owing  to 
the  compact  way  in  which  the 
fibers  are  arranged  in  the 
layer.  The  illustration  (Fig. 
348)  shows  three  of  these 
structures.  Their  myo-fibrils 
are  placed  in  radiating  plates 
or  muscle  columns,  and  the 
free  sarcoplasm  is  collected  in 
the  center  where  the  nuclei  are 
also  most  frequently  found. 

The  next  layer  is  formed 
by    a 
which 

much  thicker  than  such  struc- 
tures usually  are.  The  poison 
epithelium  is  probably  the  ac- 
tive agent  in  its  formation. 

Inside  of  the  basement 
membrane  is  found  the  poison- 
secreting  epithelium.  This 
layer  is  composed  of  a  single 
layer  of  very  high,  narrow  cells 
whose  lateral  boundaries  are 
difficult  to  see.  The  nuclei  are 
large  and  placed  well  up  from 
FIG.  348. -Section  of  the  wail  of  the  poison  gland  of  the  basement  membrane,  al- 

a  spider,  Lycosa  speciosum,  seen  in  cross  section.  .  ,         . 

mus.,  three  muscle  cells;    b.m.,  basement  mem-  though    they    are    irregular    in 

brane;   p.n.,  poison-cell   nuclei;    p.gl.  globules  of  that  respect  and  SOniC  of  them 
poison,     x  580.  x  .mi  i    • 

rest  close  to  it.    These  nuclei 

have  a  peculiar  chromatin  pattern  which  close  study  might  enable  one 
to  specifically  identify  from  the  other  nuclei  of  the  animal.  The  cyto- 
plasm is  most  markedly  striated  by  fibrillar  structures,  which  reach 
from  the  basement  membrane  to  the  distal  edge  of  the  cell.  In  fact 
they  reach  beyond  this  edge,  and  so  obscure  it  that  it  cannot  be  definitely 
determined.  Outside  of  the  cells  these  fibrils  end  in  a  loose  reticulum 
which  fills  the  lumen  and  contains  the  poisonous  fluid  in  its  meshes. 
The  poison  appears  as  a  few  yellow,  scattered  droplets  in  the  outer 


MECHANICAL  PROTECTION  AND  POISONS  383 

cytoplasm  of  the  cell.  These  grow  into  very  large  drops  which  seem 
never  to  blend,  but  to  work  their  way  individually  down  through  the 
reticulum  in  the  lumen.  Comparatively  little  of  this  matter  is  secreted 
and  it  is  possibly  only  a  part  of  the  material  which  goes  to  make  up  the 
poison  used  by  this  creature  to  kill  its  prey,  the  rest  being  a  soluble  fluid 
that  does  not  appear  in  the  usual  microscopic  preparations. 

These  cells  extend  into  the  duct  of  the  gland  as  low,  non-secreting, 
epithelial  elements,  thus  proving  the  origin  of  the  poison  cells.  The 
poison  cells  of  the  scorpion  and  allied  forms  appear  in  the  tail  and  other 
parts  of  the  body,  but  have  much  the  same  histological  and  cytological 
structure  as  those  we  have  just  examined. 

The  vertebrate  animals  are  not  without  their  poison-producing  mem- 
bers, and  some  of  these  are  among  the  most  dangerous  creatures  known, 
on  account  of  their  size  and  the  amount  of  venom  that  they  are  able  to 
inject  into  a  wound.  These  animals  also  have  the  mechanical  structures 
of  offensive  protection  remarkably  well  developed. 

Among  the  fishes  some  spines,  which  are  mesodermal  bony  structures, 
are  developed  in  connection  with  the  fins.  Most  of  these  are  not  poi- 
sonous except  for  the  slime  and  dirt  that  are  associated  with  them.  The 
sting  of  the  whip  ray,  Dysatis,  and  other  sting  rays,  makes  a  very  ugly 
wound.  It  is  barbed  and  when  broken  it  is  renewed  from  a  tissue  nu- 
cleus that  forms  new  spines,  slowly,  all  the  time. 

Some  fishes  also  have  the  first  ray  of  the  two  pectoral  and  the  dorsal 
fins  enlarged  and  barbed  to  use  as  a  weapon  of  defense.  In  some  cat- 
fishes,  as  Schilbeodes,  the  pectoral  spine  is  associated  with  a  weak  poison 
gland  (Fig.  349,  A  and  B),  which  appears  in  two  forms;  an  axillary 
gland  opening  by  a  pore  near  the  origin  of  the  fin  in  most  of  the  species, 
and  a  glandular  tissue  placed  between  the  skin  and  spine  in  those  cat- 
fishes  which  do  not  have  barbs  on  the  spines. 

The  axillary  gland  is  clearly  an  invagination  of  the  stratified  surface 
epithelium  whose  cells  are  specialized  to  secrete  poison.  They  proliferate 
and  swell  up  until  they  finally  burst,  and  the  poison  is  discharged  from 
the  pore.  As  can  be  seen  in  the  figure,  these  poison  cells  are  modified 
clavate  cells  which  occur  in  all  the  skin  of  this  and  other  fishes.  One  needs 
but  to  trace  the  row  of  clavate  cells  (Fig.  349,  A  and  B,  c.c.)  from  the 
outer  epithelium  around  and  into  the  poison  gland  to  realize  this  fact. 

In  the  fin  spines  of  such  of  the  catfishes  as  have  no  serrations  on  these 
spines,  may  be  seen  another  collection  of  the  same  poison  cells.  Here 
they  lie  between  the  integument  and  the  spine.  No  duct  is  apparent, 
and  we  must  examine  a  longitudinal  section  of  the  tip  of  the  spine  before 
the  relations  of  the  poison  cells  to  the  epidermis  can  be  understood. 

Such  a  section  (Fig.  349,  4)  shows  that  the  epithelium  on  the  end  of 
the  spine  has  been  reflected  as  a  blind  invagination  around  the  central 


384  HISTOLOGY 

bony  spine  (sp.)  and  that  the  poison  cells  are  developments  of  the  clavate 
cells  which  were  carried  in  with  this  epithelium.  In  fact,  the  one  kind 
can  be  continuously  traced  into  the  other  in  Figure  349,  A. 

Such  tissues  are  also  found  in  many  other  bony  fishes.  The  "  weavers  " 
and  their  allies  have  them,  and  in  Thalassophryne,  a  marine  form,  they 
are  developed  to  formidable  proportions. 


FIG.  349.  —  A,  tip  of  a  fin-spine  of  the  river  catfish,  Schilbeodes;  B,  section  through  the  body 
wall  to  include  the  auxiliary  poison  gland  and  the  base  of  the  pectoral  spine  of  this  fish. 
ep.,  epidermis  with  clavate  cells  (c.c.) ;  p.g.,  invaginated  epidermis  with  clavate  cells  modified 
to  become  poison  cells;  sp.,  spine;  pg.,  pigment  cells;  /.,  fat  cells.  Both  clavate  and  poison 
cells  have  double  nuclei.  (After  H.  D.  REED.) 

Passing  by  the  amphibians,  whose  so-called  poison  glands  are  used 
mostly  to  produce  offensive  odors,  we  find  that  the  reptiles  have  the 
best-developed  venomous  organs.  In  the  rattlesnake,  Crotalis  horridus, 
a  beautifully  developed  tooth  is  transformed  into  a  fang  by  the  presence 
of  a  groove  on  its  anterior  surface.  This  groove  is  turned  into  a  tube  by 
the  overlapping  of  its  edges,  leaving  an  upper  and  a  lower  aperture. 

Above,  in  the  connective  tissue  on  the  side  of  the  jaw,  an  invaginated 
region  of  the  integument  forms  a  large  gland  with  many  irregular,  al- 
veolar lobules  each  of  which  is  lined  with  a  simple  cuboidal  epithelium 
(Fig.  350,  A,  B).  The  cells  have  a  large,  somewhat  flattened  nucleus 


MECHANICAL  PROTECTION  AND  POISONS 


385 


and  show  no  vacuoles,  granules,  or  other  marked  signs  of  secretory 
activity  (Fig.  350,  B).  They  secrete  the  poison,  which  fills  the  large  cavi- 
ties of  the  gland  and  is 
sent  through  a  duct  to  the 
upper  opening  in  the  fang. 
When  the  snake  strikes,  a 
powerful  muscle  com- 
presses the  gland  and 
forces  the  venom  out  of 
the  fang  in  a  jet.  The 
fangs  are  constantly  being 
formed  as  teeth  are,  and 
several  half-developed 
ones  are  always  to  be 
found  in  the  gum,  ready 


FIG.  350. — A,  outline  of  several  lobules  of  the  poison 
gland  of  a  rattlesnake,  Crotalis  horridus,  as  they  appear 
in  a  section  of  the  gland;  B,  five  cells  from  the  epithe- 
lium of  these  lobules,  enlarged  to  show  their  finer  struc- 
ture. The  colloid  substance  distad  of  the  cells  is  the 
poison.  X  1500. 


to  come  out  and  take  the 
place  of  the  old  one  when 
it  is  lost. 

Another  reptile,  the 
Gila  monster,  has  poison 
glands  developed  in  the  integument  of  the  mouth  near  the  lower  teeth. 
A  bite  from  this  creature  is  only  venomous  when  it  succeeds  in  turning 
on  its  back  and  thus  draining  the  poison  into  the  wound. 

The  birds  and  mammals  produce  no  poisons  but  have  many  of  the 
mechanical  integumentary  forms  of  defense  and  offense.  Among  these 
may  be  merely  mentioned  teeth,  claws,  spikes,  spurs,  spines,  two  dis- 
tinct kinds  of  horns  on  top  of  the  head,  and  one  on  the  nose,  to  mention 
but  a  few  of  them.  We  shall  briefly  describe  but  two  of  these  structures, 
the  claw  as  exemplified  by  the  human  nail,  and  the  spine  of  the  porcupine. 

The  nail  is  a  cornified  outer  layer  of  stratified  epithelial  cells,  developed 
in  a  folded  area  of  the  skin  on  the  ends  of  the  fingers.  Two  regions  of 
this  fold  are  distinguished;  one,  by  the  fact  that  the  lower  posterior 
body  of  the  nail  is  formed  there  and  grows  forward  out  of  it,  and  the  other 
by  the  fact  that  the  nail  is  not  materially  added  to  as  it  grows  past  this 
region.  Both  these  regions  are  on  the  under  side  of  the  fold,  and  are 
known  as  the  nail  bed  (Fig.  351).  ,. 

The  other  side  of  the  fold  also  rests  against  the  nail,  on  its  upper 
surface,  but  this  epithelium  does  not  contribute  in  any  way  to  the  nail's 
formation  other  than  to  protect  it  from  drying  during  the  weak  begin- 
nings of  its  growth.  This  surface  is  widest  over  the  proximal  "root" 
of  the  nail,  and  forms  overlapping  ledges  on  each  side.  Its  epidermal 
layers  are  practically  the  same  as  on  the  rest  of  the  skin. 

The  basement  membrane  of  the  nail  bed  is  thrown  into  longitudinal 


HISTOLOGY 


FIG.  3  5 1 . — Longitudinal,  ver- 
tical section  of  the  young 
nail  and  nail  bed  of  an  in- 
fant, n.r.,  nail  root  or  lun- 
ula;  «.,nail;  n.6.,nail  bed. 
(From  a1  preparation  by 
DR.  H.  E.  JORDAN.) 


furrows,  and  the  layer  homologous  to  the  stratum 
Malpighi  is  developed  from  the  basal  layer.  It 
fills  the  furrows  and  covers  the  ridges,  thus 
forming  a  nearly  level  surface.  From  this  sur- 
face these  cells  are  added  to  the  nail  above,  in 
such  a  manner  that  they  overlap  each  other  like 
roof  tiles.  They  become  hardened  and  trans- 
parent. 

Claws  are  formed  much  as  are  nails,  also 
some  forms  of  horn,  as  the  cow's  horn.  The 
horn  of  the  rhinoceros  is  more  to  be  compared 
to  a  collection  of  jointly  formed  hairs  of  great 
size  and  strength.  The  spurs  on  the  wing  and 
shank  of  birds  have  a  bony  core  as  does  the 
cow's  horn. 

Even  hairs  can  be  specialized  into  a  set  of 
very  formidable  weapons.  In  the  porcupine, 
Erinaceus,  they  are  immensely  long  and  strong, 
and  have  pointed  and  barbed  ends  which  are 
very  injurious  to  their  victims.  On  account  of 
its  size  this  hair  is  developed  by  an  exterior  or 
lateral  hardening  of  the  superficial  layers  of 
stratified  epithelium  on  a  deep-set  papilla  (Fig. 
352).  In  this  respect  they  resemble  a  feather 
somewhat,  especially  as  the  papillar  epithelium 
is  thrown  into  longitudinal  ridges  to  strengthen 
the  shaft  and  form  the  distal  barbs. 

Technic.  — The  cutting  of  sections  of  the 
materials  mentioned  in  this  chapter  is  often 
very  difficult  owing  to  the  extreme  hardness 
and  toughness  of 
some  of  them. 
Certain  small  or 
delicate  hairs, 
feathers,  and  nails 
may  be  ignored 
and  sections  cut 
as  usual  after  any 
good  fixation  in 
the  usual  fluids. 
With  the  larger 

and  harder  kinds,  FlG-  352-— Transverse  section  of  the 
i  ,  ,  developing  spine  of  a  porcupine, 

the     best     general        Erinaceus.    (After  DAVTES.) 


LUBRICATING    TISSUES  387 

aim  is  to  fix  in  some  fluid  that  will  harden  the  elements  as  little  as 
possible  and  then  try  to  embed  the  tissue  at  once  and  as  quickly  as 
is  possible.  Under  favorable  circumstances  this  may  result  in  the 
least  amount  of  hardening,  and,  if  the  knife  is  very  sharp,  fairly  good 
sections  may  be  obtained. 

If  the  horny  matter  is  exceedingly  refractory  it  is  better  not  to  try  to 
cut  it  in  situ,  but  to  separate  it  from  the  surrounding  tissues,  and  cut  them 
separately,  the  soft  parts  in  the  ordinary  way,  and  the  horn  or  nail 
after  some  of  the  macerating  and  softening  methods  such  as  40  per 
cent  potash  solution  or  strong  mineral  acids  or  javelle  water. 

LITERATURE 

Several  of  the  papers  mentioned  after  the  last  part  will  be  useful  guides  for  this.    Among 
those  which  deal  more  especially  with  the  poison-secreting  tissues  are  the  following :  — 
BORDAS,  L.     "Recherches  anat.,  histologiques,  et  physiol.  sur  les  glandes  venimeuses  ou 

glandes  des  cheliceres  des  malmignattes  (Lalrodecties  i3-guttatus  Rossi),"  Ann.  Ac. 

Nat.  Zool.  (9),  Ann.  79,  pp.  147-164,  i  pi.,  4  figs.,  1905. 
GUNTHER,  A.     "On  a  Poison  Organ  in  a  Genus  of  Batrachoid,"  Proc.  Zool.  Soc.,  Lon., 

1864,  p.  157. 
PARKER,  W.  N.     "On  the  Poison  Organs  of  Trachinus,"  Proc.  Zool.  Soc.,  Lon.,    1888, 

P-  359- 
WALLACE,  LOUISE  B.     "The  Structure  and  Development  of  the  Axillary  Glands  of  Ba- 

trachus,"  Journ.  Morph.,  1893,  Vol.  VIII,  p.  563,  pi.  27. 
REED,  H.  D.     "The  Poison  Glands  of  Noturus  and  Schilbeodes,"  American  Naturalist, 

Vol.  XLI,  Nr.  489,  pp.  553-566,  1907. 


INTEGUMENT,   LUBRICATION 

The  bodies  of  most  organisms  are  lubricated  with  some  fluid,  and  this 
fluid  also  acts  in  other  capacities  in  many  cases;  as  a  preservative,  a 
cleanser,  a  food-gatherer,  or  even,  deserting  its  original  duty,  it  may  be 
developed  as  a  poisonous  substance  or  a  foul-smelling  or  attractive- 
smelling  material  accordingly  as  it  best  serves  a  purpose.  Sometimes 
the  fluid  is  used  to  lubricate  some  particular. portion  of  the  body-surface 
or  it  may  be  found  on  the  entire  surface. 

The  fluid  may  be  produced  from  all  parts  of  the  body-surface  or  by 
some  parts  of  it  only,  which  may  be  further  defined  from  the  rest  by  being 
invaginated  into  glands  that  pour  out  their  secretions  on  sucn  parts 
of  the  integument  as  require  it.  We  may  distinguish,  according  to  the 
kind  of  fluid  that  is  produced,  three  principal  kinds  of  lubrication  tissues. 

i.  A  tissue  that  produces  a  slimy  material  called  mucin.  This  sub- 
stance has  a  definite  chemical  basis  and  is  a  product  that  can  be  used 
for  other  internal  processes  or  external  processes  than  those  of  lubrication, 
such  as  preservation  of  the  integument  by  the  retention  of  water,  cleans- 
ing, the  collecting  of  food  particles,  the  making  of  cocoons  for  the  eggs, 


388  HISTOLOGY 

and  the  making  of  dwelling  cavities  for  the  entire  animal.  The  specific 
cells  of  this  tissue  are  not  to  be  confounded  with  those  of  the  serous  tis- 
sues which  produce  their  secretion  in  a  similar  way.  This  method  of 
lubrication  is  mostly  characteristic  of  animals  that  live  in  the  water 
although  many  of  these  spend  large  periods  of  their  lives  in  the  air. 
Here,  however,  they  must  be  in  a  damp  location,  for  it  is  true  of  mucus 
that  it  will  not  withstand  a  thorough  drying. 

2.  A  tissue  that  secretes  an  oily  substance  called  the  sebaceous  fluid, 
whose  primary  use  is  to  lubricate  the  skin  and  its  appendages,  and  to 
preserve  them  from  the  effects  of  drying.     Among  its  principal  secondary 
uses  is  the  production  of  odors. 

This  tissue  is  found  in  those  animals  that  live  in  the  air,  although 
it  has  persisted  in  some  of  those  that  have  later  adapted  themselves 
to  the  water.  The  only  alternative  open  to  an  animal  that  lives 
exposed  to  the  drying  effects  of  the  air  and  sun  is  to  have  a  hard, 
impervious  covering  of  cuticle  as  in  some  of  the  Insecta,  Crustacea, 
and  lower  vertebrates.  These  animals  dry  very  rapidly  when  confined 
without  a  supply  of  water,  showing  that  their  cuticle  does  not  entirely 
protect  them. 

3.  A  tissue  that  furnishes  a  "serous"  secretion  for  lubrication  pur- 
poses.   This  form  is  somewhat  rare,  and  is  usually  found  in  parts  where 
the  surface  to  be  lubricated  is  internal,  as  in  the  joints  between  the  bones, 
or  semi-internal,  as  on  the  surface  of  the  eye.    In  this  last  case  the  effects 
of  drying  are  avoided  by  the  frequent  renewal  of  the  serous  fluid  by  the 
act  of  "winking"  or  rapid  closing  of  the  eyelid,  thus  carrying  the  fluids 
over  the  surface  of  the  eyeball. 

These  fluids  are  of  several  kinds,  and  originate  in  several  different 
secretory  surfaces  or  in  glands  that  communicate  with  these  surfaces. 
Some  lubricating  glands  are  distributed  over  certain  surfaces  of  the 
mammalian  body  and  are  called  the  sweat  glands.  Others  are  confined 
to  particular  regions  and  perform  very  special  duties  as  the  wax  glands 
of  the  ear  tube.  To  cover  the  field  thus  laid  out  we  shall  examine  the 
following  cases. 

i5/,  a  general  lubrication  of  the  body  of  a  water  animal  with  mucus, 
accompanied  with  the  removal  of  dirt  and  collection  of  food:  the  clam, 
Mya  arenaria. 

2d,  the  production  of  mucin  by  a  land  snail,  Mesodon,  from  cells  of 
extreme  specialization. 

36?,  a  general  and  complete  lubrication  of  the  body,  and  the  animal's 
habitat  with  mucus :  the  earthworm,  Lumbricus. 

4th,  a  general  lubrication  of  the  body-surface  of  an  animal  living  in 
the  air,  with  mucus  (mixed  with  a  poisonous  or  offensive  substance): 
the  toad,  Bufo  Americanus. 


LUBRICATING    TISSUES 


389 


5//z,  lubrication  of  parts  of  the  body  with  an  oil  produced  by  a 
gland :  the  sebaceous  glands  of  the  cat  and  the  oil  glands  of  the  fowl. 

6th,  the  lubrication  of  the  eye  in  the  alligator  and  mouse. 

7th,  the  lubrication  of  the  joints  of  a  cat's  bones  by  the  synovial  fluid. 

8th,  the  sweat  glands  and  wax  glands  of  the  mammals. 

The  Mucous  Tissues  of  the  Clam,  M ya  arenaria.  —  If  the  mantle-fold 
of  a  living  example  of  this  mollusk  be  drawn  apart  and  the  surface  of 
the  inner  sides  of  the  mantle  as  well  as  the  surface  of  the  foot  be  exam- 
ined, it  will  be  noticed  that  bits  of  paper,  particles  of  dust,  and  any  other 
small  objects  that  may  be  dropped  on  this  surface  will  move  along, 
always  in  a  definite  direction  or  path.  A  mapping  out  of  all  the  paths 
so  determined  will  show  that  they  form  larger  courses,  of  various  curves 
and  straight  runs,  all  tending  toward  and  uniting  to  end  at  the  labial 
palps,  and  then  passing  from  these  to  the  mouth. 

A  section  of  one  of  these  regions  (Fig.  353)  will  show  two  facts;  the 
presence  of  many  mucous  cells  among  a  vastly  greater  field  of  ciliated 
cells  which,  in  life,  are  always 
moving  their  cilia  at  any  one 
point  in  the  same  direction,  and 
that  direction  the  same  as  one  of 
the  paths.  Thus,  it  is  the  cilia 
that  furnish  the  motive  power, 
but  the  food  and  the  dirt  par- 
ticles would  not  cling  to  the  cilia 
alone  especially  under  water. 
Here  the  mucous  cells  of  the  in- 
tegument play  their  part  by  pro- 
viding a  thin  viscid  covering  to  the  ciliated  surface.  To  this  sticky 
surface  the  particles  of  food  as  well  as  the  particles  of  sand,  dirt, 
etc.,  in  the  water  stick  and  the  whole  mass  is  thus  carried  along  and 
finally  engulfed  in  the  mouth.  The  lubrication  cells  (which  here  pro- 
duce mucus)  do  comparatively  little  of  what  we  might  strictly  call 
lubrication.  This  is  especially  true  of  some  of  the  fixed  plecopods,  as 
the  oyster,  in  which  there  is  no  active  foot  and  the  animal  scarcely 
moves  in  the  quiet  recess  of  its  shell.  In  other  forms  that  lead  an 
active  life,  especially  the  kinds  that  burrow  rapidly  in  the' sand,  as 
Unio,  Mactra,  or  Ensatella,  there  is  a  considerable  amount  of  contact 
and  abrasion  between  the  strong,  muscular  foot,  the  mantle  and  other 
parts,  and.  here  the  mucus  acts  as  a  viscid,  sliding  buffer  between  the 
delicate  surfaces.  In  other  words  it  acts  as  a  true  lubricant.  Mya  is 
a  form  that,  so  far  as  motion  and  a  consequent  lubrication  is  concerned, 
occupies  a  position  about  midway  between  the  two. 

The  mucous  cells  lie  singly  in  different  parts  of  the  epithelium  and 


FIG.  353.  —  Body  epithelium  of  the  clam,  Mya. 
The  majority  of  the  cells  bear  the  numerous 
cilia  on  their  distal  surface,  b.m.,  basement 
membrane;  mu.c.,  mucous  cells  with  round 
bodies  and  flat,  crescentic  nuclei. 


390 


HISTOLOGY 


are  much  more  abundant  in  some  parts  than  in  others.  They  usually 
are  cells  of  about  the  same  length  as  the  surrounding  ciliated  cells  al- 
though they  are  by  far  wider  owing 
to  the  distended  cell  contents.  This 
latter  consists  of  mucin  granules 
which  fill  the  entire  cell,  packed  full, 
and  gives  it  the  round  shape. 

The  nucleus  is  crowded  to  the 
cell-wall  and  becomes  much  flat- 
tened. It  lies  against  the  wall,  but 
in  this  particular  case  it  may  as 
often  lie  against  the  side  wall  as 
against  the  bottom,  where  it  usually 
lies  in  most  other  mucous  cells. 
The  nuclear  content  is  very  much 
compressed.  The  mucous  cells  bear 
no  cilia,  and  their  secretion  is  dis- 
charged as  a  swelling  mass  of  mu- 
cus of  thread-like  form.  As  to  the 
periodicity  of  this  discharge  or  the 
renewal  of  the  cells  no  facts  were 
observed. 

Some  of  these  cells  are  so  large, 
in  this  and  other  mollusks,  that  they 
cannot  be  contained  in  the  epithe- 
lium. In  this  case  the  proximal  part 
of  the  cell  grows  or  pushes  down  and 
lies  in  the  connective  tissue  beneath 
the  epithelium. 

Many  of  the  mucous  cells  of  a 

F'LS7dge  oT?  ,rnlu".a'g:     "»»  SnaU>  "««*>»  "ide«^'  5h°W 

cell  body  filled  with  secretion;  n.,  neck    this  specialization  at  its  extreme,  and 

through  which  secretion  is  discharging;  w.,     m  Figure    -^  Qne  Qf  thege  ceUs  and 

secretion  of  this  and  another  similar  cell  to  .  ^>j-r 

left;  nu.,  nucleus;  ep.,  surface  epithelium     the    Sides  of   tWO    Others  are  pictured 

to  which  this  cell  belongs  morphologically.    to  show  their  structure  and  relations 

to  the  ordinary  epithelium  cells  be- 
tween which  they  open.  As  in  all  mucous  cells,  the  nucleus  lies  flat 
against  the  cell- wall  and  in  this  case  against  the  bottom  of  the  cell.  It 
is  large  and  its  chromatin  is  compacted  so  that  its  particles  form  an 
almost  solid  mass. 

This  principle  is  carried  one  step  further  in  the  nidamental  cells  of 
the  leech,  an  account  of  which  will  be  found  in  Chapter  XXII,  and 
should  be  glanced  over  in  this  connection. 


LUBRICATING   TISSUES 


39' 


Mucous  Lubricating  Cells  of  the  Epidermis  of  the  Earthworm.  — The 
slime  cells  are  well  represented  by  the  mucin-producing  cell  of  the  epi- 
dermis of  the  earthworm.  These  cells  may  be  considered  as  certain  of 
the  simple  epithelium  cells  that  cover  the  surface  of  the  body,  and  that 
have  been  differentiated  in  structure  so  that  they  are  able  to  secrete  the 
specific  substance  called  mucin.  The  mucin  is  produced  as  good-sized, 
hard  granules  in  the  cytoplasm  of  the  cell,  and  has  the  property  of  mixing 
with  water  into  a  jelly-like,  viscid  mass  of  many  times  its  original  size. 
The  substance  can  be  identified  easily  in  a  number  of  ways  by  its  staining 
properties  and  by  its  solution  reactions  in  different  media.  The  granules 
of  mucin  are  produced  in  the  cytoplasm  of  the  cell  and  in  regions  of  this 
cytoplasm  that  are  arranged  in  more  or  less  straight  rows  reaching  from 
the  proximal  to  the  distal  end  of  the  cell.  The  substance  of  the  mucin 
comes  from  the  blood  as  a  fluid,  invisible  to  the  observer,  and  this  fluid 
is  converted  into  the  mucin  by  the  activities  of  the  cytoplasm  in  ways 
that  we  cannot  as  yet  understand.  They  appear  as  granules  of  a  small 
size  and  grow  to  then- 
full  size,  which  is  con- 
siderable (Fig.  355). 
When  fully  formed  the 
granules  fill  the  entire 
cell  to  such  an  extent 
that  it  is  distended  to 
many  times  its  original 
bulk.  The  reason  of 
this  great  distention  is 
that  the  cell  matures 
all  of  the  mucin  gran- 
ules at  once.  This  is 
not  true  of  some  other 


FIG.  355.  — Vertical  section  of  a  bit  of  epidermis  of  the  earth- 
worm. Shows  four  mucous  cells  in  different  stages  of  secre- 
tion, mostly  later  stages,  cu.,  cuticle  with  two  pores  shown, 
from  one  of  which  mucus  is  emerging.  X  noo. 


mucin-producing  cells  which  contain  the  granules  in  all  stages  of 
maturity,  and  which  are  constantly  giving  off  some  that  are  ripe. 
No  matter  how  distended  the  cell  may  become  it  never  moves  proxi- 
mally  out  of  the  line.  When  the  earthworm's  cell  is  ripe  it  dis- 
charges its  contents  in  a  very  short  time  and  is  then  seen  in  a  state 
of  collapse.  Some  peculiar  differentiated  areas  in  the  cytoplasm  now 
show  the  positions  that  were  occupied  by  the  granules,  and  the  nucleus, 
instead  of  being  crowded  down  against  the  bottom  of  the  cell  and  flat- 
tened out,  has  arisen  to  a  point  about  one  fourth  of  the  height  of  the  cell 
and  has  assumed  a  round  contour  and  the  characteristic  chromatin  and 
nucleolar  conditions  of  functional  activity.  The  cell  begins,  after  a  very 
short  rest,  to  secrete  mucin  again. 

The  mucin  is  extruded  from  the  cell  through  the  fine  pore  in  the 


392 


HISTOLOGY 


cuticle  that  is  found  opposite  each  gland-cell.  It  is  used  not  only  to 
lubricate  the  body  but  to  permanently  line  the  tube  in  which  the  animal 
lives  with  a  mucous  covering. 

These  cells  are,  in  some  stages,  very  much  like  a  number  of  the  other 
epidermal  cells  in  the  epithelium  of  the  earthworm  that  are  also  modi- 
fied for  the  purpose  of  secretion.  This  other  kind  of  cell,  however,  pro- 
duces a  very  different  kind  of  secretion,  and  it  might  well  be  studied  as  a 
type  of  the  so-called  serous  cell  or  albumen  cells  of  the  animal  tissues. 
It  differs  in  the  earthworm  from  the  mucin  cells  in  several  ways.  Most 
important  is  the  staining  reaction  of  the  secretion.  This  and  chemical 
tests  show  that  it  is  a  different  substance  from  mucin  although  it  is  pos- 
sible, considering  the  derivation  of  the  two  cells  and  the  similarity  of  the 
manner  in  which  they  secrete,  that  both  cells  and  secretions  were  derived 
from  a  common  kind  or  that  one  was  derived  from  the  other. 

The  simple,  unicellular,  mucous  gland  may  thus  be  followed  through 
a  series  of  forms  of  increasing  specialization  terminating  with  the  extreme 
form  found  in  the  leech  which  is  discussed  in  another  connection  (Chapter 
XXII).  This  single  mucous  cell  is  found,  in  some  very  rare  cases,  in  an 
epithelium  that  has  become  stratified,  and  Figure  356  shows  a  case  of 
this  kind  in  a  section  of  part  of  the  alligator's  conjunctiva.  Here  a 

single  mucous  cell  is  to  be  seen 
rising  from  the  basement  mem- 
brane through  the  several  stratified 
layers,  and  opening  freely  to  dis- 
charge its  secretions.  Similar  con- 
ditions are  found  in  the  skin  of 
many  fishes. 

The  mucous  cell  is  found  not  only 
in  the  single-celled  forms  dealt  with 
in  the  above  paragraphs,  but  also 
collected  into  groups  that  are  mostly 
invaginated  into  glands  of  varying 
complexity.  Such  a  gland  may  be 
found  to  be  derived  from,  and  open- 
ing on  to,  a  simple  epithelium  or  a 
stratified  epithelium.  The  former 


FIG.  356.  —  Vertical  section  through  a  bit  of 


should  be  considered  a  more  primi- 
the"  stratified  epithelium  lining0  the" alii-    tive  form,  perhaps,  and  can  be  easily 

gator's  conjunctiva.     Shows   one   mucous      seen  Jn  tne  mUCOUS    Secreting  glands 
cell    extending    through    the    epithelium.          ...  .  .  .... 

x  880.  which  open  into  the  small  intestine. 

These  are   known  as  the  duodenal 

glands  or  Brunner's  glands,  and  the  simple  epithelium  which  lines  them 
can  be  traced  through  their  ducts  and  into  the  simple  columnar  epithe- 


LUBRICATING    TISSUES 


393 


Hum  that  lines  the  whole  intestine.  Another  example,  which  does  not 
so  well  illustrate  the  principle,  is  seen  in  the  sac-shaped  mucous  glands 
which  open  out  on  to, 
and  are  derived  from, 
the  olfactory  epithe-  "•*-•«-. 

Hum  of  the  nasal  cav- 
ity. This  epithelium 
is  hardly  simple,  how- 
ever, and  a  still  sim- 
pler and  better  «.w».--^=^  ^.^  \~ 
example  is  seen  in  Pff-c-^^  -  ^^/'$$M  iiii'^'^K 

,  v  >il  N-         -»3fW?jB»  ' '  <rf 

the  many  tubular 
mucous  glands  that 
open  into  the  lumen 
of  the  uterus  of  many 
mammals,  as  the 
cat. 

Of  mucous  glands 
which  are  derived 
from  a  stratified  epi- 
thelium we  have  many 
examples,  and  such 
glands  may  be  noticed 
in  the  mucous  glands 
of  the  Amphibia. 
These  are  each  com- 
posed of  a  few  typical 

mucous  cells  that  are  in  communication  with  the  exterior  by  means  of 
a  short  duct  that  is  also,  of  course,  constructed  of  the  epithelial  cells. 
In  many  cases  the  duct  is  constructed  of  a  single  cell,  and  the 
cells  immediately  connecting  it  with  the  body  of  the  gland  are  specialized 
to  control  the  flow  of  the  secretion.  When  the  epidermis  is  shed  in  the 
periodical  molts  of  the  animal,  the  cell  or  cells  that  form  the  duct  are 
shed  with  it.  This  form  of  gland  is  well  shown  in  Figure  357  from  the 
toad.  The  gland-cells  in  this  and  the  following  case  appear  to  represent 
the  basal  layer  of  the  stratified  epithelium  from  which  the  gland  was 
developed. 

Such  a  simple  form  of  invaginated  gland  is  also  represented  in  the 
higher  forms  (mammals)  by  the  mucous  glands  found  in  the  posterior 
portion  of  the  tongue.  Here  a  large  number  of  glands  like  those  found 
in  the  salamander  are  joined  together  and  empty  the  secretion  out  through 
a  common  duct  that  has  many  branches.  Still  larger  collections  of  the 
same  structures  form  some  of  the  large,  salivary  glands  that  lie  far  back 


FIG.  357.  —  A  many-celled,  succular,  mucous  gland  from  the 
skin  of  a  toad,  str.ep.,  stratified  epithelium  on  surface; 
sec.ep.,  secreting  epithelium  in  gland;  b.m.,  basement  mem- 
brane; d.,  duct;  pg.c.,  pigment  cells,  x  520. 


394 


HISTOLOGY 


between  the  muscles  and  the  integument  on  the  side  of  the  head,  where 
they  form  important  anatomical  features.  See  the  chapter  on  alimentary 
tissues  of  the  animals  in  question  (see  Chapter  XV). 

Sebaceous  or  Oil-Lubricating  Tissues.  — The  second  class  of  lubri- 
cating tissues  is  found  almost  exclusively  in  the  integument  of  the  higher 
vertebrates  (mammals  and  birds),  where  there  is  developed  a  type  of 

gland  that  is  used  to  produce  a  fluid 
for  lubrication.  This  type  is  essen- 
tially different  from  the  mucous  type 
in  most  important  respects.  It  pro- 
duces an  oil  instead  of  mucin.  The 
cell  is  nearly  always  destroyed  by  the 
act  of  secretion  instead  of  repeating 
its  duty  a  number  of  times.  Also  the 
cells  that  do  this  always  occur  in  a 
stratified  epithelium  in  which  they  can 
be  more  conveniently  renewed  than 
in  a  simple  layer  of  cells.  The  cells 
that  do  this  work  never  operate  alone 
but  always  in  some  considerable  ex- 
tent of  epithelium  which  is  usually 
invaginated  to  form  a  gland.  This 
form  of  tissue  is  sometimes  secondarily 
adapted  to  produce  an  oil  that  is  at- 
tractively or  offensively  odorous  (see 
next  part).  It  is  used  most  com- 
monly to  lubricate  the  hair  in  mam- 
mals, and  the  feathers  in  the  birds, 
and  our  first  study  may  very  prop- 
erly be  on  the  common  sebaceous 
gland  of  the  cat's  skin. 

Sebaceous  or  Lubricating  Gland  of 
Mammals;    from    the  Lip   of  a   Cat 
(Fig.  358).  —  These  glands  are  found 
clustered    around    the    hairs   at   the 
FIG.  358.  — Root  of  a  small  hair  in  the  lip  Pomt  where  they  are  about  to  leave 
of  a  cat  to  show  the  sebaceous  gland  the    skin   and   where   the   sheath   is 

(seb.gl.);  sh.,  shaft  of   hair;   «.,  neck  or       ,    .    ,         ...          .  ,     ,  , 

duct  of  gland  in  which  the  secretion  col-  plainly  distinguishable  as  an  mvagi- 
lects.    (Taken  from  a  preparation  by  nated  continuation  of  the  epidermis. 

Being    glands    that    are    invaginated 

from  this  sheath,  we  may  say  that  they  are  invaginations  of  the 
stratified  epithelium. 

The  stratified  epithelium  is  carried  into  the  invagination,  but  is  so 


seb.  gl. 


LUBRICATING    TISSUES  395 

much  thickened  inside,  that  it  practically  fills  it  up  and  leaves  very  little 
lumen  to  be  seen.  This  gives  the  gland  a  solid  appearance  with  the  basal 
cells  of  the  stratified  epithelium  at  the  fundus.  These  basal  cells  appear 
to  be  in  the  same  condition  that  they  exist  in  other  parts,  but  the  distal 
layers  become  larger  as  they  approach  the  surface  of  the  lumen  or,  if 
there  is  no  lumen,  the  neck  of  the  gland.  Knowing  that  stratified  epi- 
thelium proliferates  from  the  basal  layer,  we  can  follow  the  history  of 
these  cells  of  the  sebaceous  glands  and  see  that,  after  a  cell  has  been 
divided  off  from  the  basal  layer  and  started  on  its  journey  towards 
the  surface,  it  undergoes  changes  that  are  very  different  from  the  well- 
known  changes  seen  in  other  and  unspecialized  parts  of  the  outer  epi- 
thelium of  man. 

Instead  of  forming  keratin  it  goes  through  a  process  of  vacuolization 
in  which  the  vacuoles  are  filled  with  the  oil  which  has  been  elaborated 
at  the  expense  of,  and  through  the  destruction  of,  the  cytoplasm.  As 
the  elaboration  of  oil  continues,  the  nucleus  disintegrates  so  that  the 
production  of  oil  in  these  glands  results  in  the  destruction  of  the 
cell. 

The  oil  drops  begin  to  appear  in  the  part  of  the  cytoplasm  next  the 
nucleus,  as  numerous  and  small  vacuoles.  They  rapidly  increase  in  size 
until  they  fill  the  cytoplasm,  and  by  the  time  that  the  cell  has  reached 
the  surface  of  the  epithelium,  or  neck  of  the  gland,  it  appears  as  a  mass 
of  oil  drops  inclosed  in  a  sac,  the  cell-membrane,  and  containing  the 
remains  of  the  nucleus.  A  number  of  these  ripe  cells  collect  at  the  fundus 
and  rupture  to  form  the  glandular  discharge.  The  secretion  thus  contains 
the  degenerated  nuclei  and  cell-membranes  of  the  cells  that  produced  it 
and  which,  we  can  now  see,  were  completely  sacrificed  in  the  production 
of  one  portion  of  the  secretion  of  the  gland.  This  is  not  the  case  with 
most  glands. 

Sebaceous  or  Lubricating  Glands  in  the  Birds;  Oil  Glands  of  the 
Chicken.  —  Here  are  found  two  large  groups  of  sebaceous  glands  lying 
parallel  with  one  another  (or  nearly  so),  and  all  the  glands  of  each  group 
opening  into  a  single  cavity,  itself  an  invagination  of  the  integument 
on  the  rump  of  the  animal.  Each  of  these  glands  is  long  and  tubular 
in  shape,  with  a  wide  and  clearly  defined  lumen  which  is  of  equal  width 
for  its  entire  length.  The  gland  shows  no  differentiation  into  the  neck 
and  fundus.  Its  long  sides  are  everywhere  lined  with  an  epithelium 
that  is  very  evidently  a-  stratified  epithelium  of  some  ten  or  twelve 
irregular  layers  (Fig.  359).  These  layers  all  come  from  a  basal  layer 
that  proliferates  them  exactly  as  in  an  ordinary  stratified  epithelium, 
with  the  important  exception  that  no  amitotic  divisions  take  place  in 
the  cells  that  are  moving  to  the  outer  surface. 

The  first  two  rows  of  these  cells  show  no  change  except,  perhaps,  an 


396 


HISTOLOGY 


increase  in  size.  When  they  have  attained  to  the  third  row,  however, 
it  can  be  seen  that  a  number  of  vacuoles  form  in  the  cytoplasm  and  these 
continue  to  rapidly  grow  as  the  cell  moves  outward,  until  at  the  eighth 

or  tenth  row,  or  layer, 
the  cell  is  many  times 
larger  on  account  of 
the  increase  in  the 
size  of  the  vacuoles, 
and  the  nucleus  be- 
gins to  shrivel  and  to 
lose  its  structure. 

The  vacuoles  are 
filled  with  an  oil;  in 
the  two  outer  rows 
the  cell  structure 
completely  breaks 
up,  leaving  only  the 
oil  containing  some 
remnants  of  the  nu- 
cleus and  cytoplasm. 
The  lubricant  has 
been  formed,  but, 
unlike  the  mucous 
cell,  the  process  can- 
not be  repeated  by 
the  cell  because  it 
has  been  destroyed 
in  the  process.  Sev- 
eral other  forms  of 
oil-secreting  glands 
are  to  be  found 
among  the  reptiles, 
birds,  and  mammals,  and  some  of  these  have  such  a  secondary  use  that 
they  are  treated  of  in  the  next  part. 

The  lubrication  of  the  eye  in  such  vertebrate  animals  as  live  in  the 
air  is  somewhat  complex.  Two  types  of  gland  tissue  are  employed  in 
this  function  besides  the  few  true  mucous  cells  that  occur  in  the  con- 
junctiva. The  first  of  these  is  a  tissue  that  produces  a  peculiar  oily  fluid 
and  is  represented,  in  a  simple  form,  by  the  glandular  epithelial  surface 
found  on  the  conjunctiva  of  the  alligator  (Fig.  360). 

The  epithelium  is  a  thin,  stratified  form,  and  its  layers  are  but  two 
or  three  in  number  in  a  young  animal  of  eighteen  inches  in  length.  It 
is  the  outer  layer  that  is  interesting  because  it  is  not  composed  of  dead 


FIG.  359.  —  A  vertical  section  of  a  portion  of  the  secreting  epi- 
thelium that  lines  the  tubular  oil  glands  of  a  chicken.  Base- 
ment membrane  below,  distal  surface  and  lumen  above. 


LUBRICATING   TISSUES 


397 


sec.  c.  \ 


str.e 


6.W.1- 


thelium;   sec.c.,   distal  layer  of 
secreting  cells.     X  880. 


cells  as  in  the  surface  layer  of  most  stratified  epithelia,  but  of  a  layer  of 

columnar  secreting  cells  of  great  activity. 
These  columnar  cells  give  the  epithelium 

its    name    "pseudostratified"    and,    besides 

their  long  bodies  packed  with  the  secretion 

product,  they  are  distinguished  by  having  a 

slightly  smaller  and  more  chromatic  nucleus 

which  lies  in  the  proximal  end  of  the  cell. 

The  secretion  of  these  cells  is  slowly  dis- 
charged from  the  distal  surface,  and  used  as 

a  lubricant  for  the  eyelid. 

It  will  be  observed  upon  examining  this 

epithelium   that   the   secreting   cells,   unlike 

those  of  nearly  every  other  kind  of  gland,  do    FIG.  360.— Part  of  the  epithelium 

not  lie  in  close  contact  with  a  blood  supply.      *-«£  ^~n?±" 

They  must,  therefore,  receive  their  food  sup-      brane;   str.ep.,  stratified   epi 

ply  through  the  efforts  of  the  cells  that  lie 

between  them  and  the  blood.     This  involves 

unnecessary  labor  and  is  rarely  seen,  the  proximal  surface  of  all  such 

cells  usually  lying  directly  against  the  thin  wall  of  an  arterial  blood 

supply. 

Another  form  of  this  kind  of  lubricating  tissue  is  found  in  the  tear 

gland  of  the  mouse.  The 
secreting  epithelium  is  sim- 
ple in  this  case  and  has 
been  invaginated  into  a 
large,  compound,  saccular 
gland,  a  portion  of  whose 
secreting  epithelium  is  rep- 
resented by  Figure  361. 

At  first  sight  these  cells 
seem  to  correspond  with 
those  of  the  sebaceous  tissue 
seen  in  the  chicken's  rump 
glands,  both  as  to  character 
of  secretion  and  method  of 
producing  it  by  the  sacrifice 
of  a  succession  of  cells.  The 
latter  is  easily  disproved  by 
noting  that  the  cells  of  the 
tear  gland  form  a  single 

layer,  and  that  there  is  no  evident  means  of  renewal.    The  idea  that 

the  method  of  production  is  much  the  same,  however,  holds  true  in 


FIG.  361.  —  Part  of  a  section  through  an  acinus  of  the 
tear  gland  of  a  mouse.  The  cells  show  a  vacuolated 
cytoplasm  which  discharges  its  secretion  into  the 
lumen.  X  800. 


398 


HISTOLOGY 


part,  in  that  the  secretion  appears  as  little  globules,  or  in  vacuoles,  in 
the  proximal  end  of  the  cell  and  either  moves  through  or  with  the  cyto- 
plasm toward  the  distal  end,  where  it  ruptures  its  bounds  and  is  set  free 
in  the  lumen,  together  with  a  disintegrating  portion  of  the  cytoplasm. 
Notwithstanding  this  probable  cytoplasmic  movement,  the  single  nucleus 
remains  in  its  proximal  position  in  the  cell. 

The  secretion  stains  black  in  osmic  acid,  but  is  not  a  real  fat  because 
it  dissolves  in  water. 

Another  form  of  serous  lubrication  takes  place  in  the  cavity  between 
two  bones  that  form  a  joint  in  the  vertebrate  animals.  The  membrane 
which  closes  in  this  joint  cavity  at  the  sides  is  known  as  a  synovial  mem- 
brane, and  through  its  agency  is  produced  the  synovial  fluid. 

The  real  secretory  cells  that  produce  the  synovial  fluid  are  not  clearly 
defined  from  the  connective  tissues  that  make  up  the  bulk  of  this  mem- 
brane. In  larger  joints, 
the  membrane  is  evagi- 
nated  into  the  cavity,  as 
a  series  of  short  papillae 
in  some  forms,  or  as  a 
single,  septum-like  lamella 
which  reaches  as  far  as 
the  danger  of  being 
pinched  between  the 
bones  will  permit.  The 
synovial  membrane  of 
the  cat  is  amplified  in 
this  latter  fashion,  and 
the  illustration,  Figure 
362,  shows  a  vertical 
section  through  a  portion 
of  this  lamella  near  its 
inner  boundary. 

The  connective  tissue 
that  forms  the  central  part  of  this  organ  is  arranged  as  a  rather 
wide-meshed  reticulum.  Blood  vessels  and  lymphatics  pass  through 
this  reticulum,  and  from  them  is  derived  the  water  and  the  very  small 
proportion  of  other  matter  which  makes  up  the  synovial  fluid.  The 
synovial  cells  that  secrete  the  fluid  must  exert  some  specific  influ- 
ence on  the  selection  of  its  proper  constituents.  They  possess  a 
cytoplasmic  body  of  some  size  and  solidity,  and  their  position  on  the 
surface  gives  them  an  epithelial  arrangement. 

The  sweat  glands  of  some  mammals  are  perhaps  to  be  considered 
here.  While  they  perform  no  real  function  of  mechanical  lubrication, 


bl.  \M. 


FIG.  362 .  —  Part  of  a  vertical  section  through  the  synovial 
lamella  from  a  cat's  joint,  bl.ca.,  blood  capillary  con- 
taining a  corpuscle. 


LUBRICATING    TISSUES 


399 


they  pour  out  a  fluid  that  is  used  principally  to  keep  the  skin  moist  and 
to  reduce  surface  heat  by  evaporation.  These  glands  are  simple  tubular 
glands  which  are  continuous  with  the  basal  cells  of  the  stratified  epithe- 
lium. The  duct  reaches  up  through  the  outer  layers  of  this  epithelium 
to  open  at  the  surface. 

The  gland  itself  passes  down  into  the  connective  tissue  of  the  skin 
and  ends  as  a  coil  of  somewhat  similar  structure.  This  structure  con- 
sists, in  the  duct,  of  two  or  three  layers  of  cells  which  constitute  a  weakly 
stratified  layer.  The  lumen  thus  formed  is  bounded  by  a  cuticle  belong- 
ing to  the  inner  layer  of  cells.  A  distinct  basal  membrane  appears 
between  the  epithelium  and  the  surrounding  connective  tissue,  and  on 
this  are  a  few  longitudinal  smooth  muscle  fibers. 

The  lower  coiled  part  of  the  tube  is  lined  by  a  single  layer  of  cubical 
or  columnar  cells  which  do  the  active  secretion.  Several  phases  of  activ- 
ity may  be  detected  in  them,  and  it  is  known  that,  while  under  ordinary 
conditions  they  secrete  an  oily  material  to  lubricate  the  skin,  when  ex- 
cited by  the  proper  nervous  stimuli  and  blood  supply,  these  glands  pour 
out  the  watery  sweat,  probably  in  addition  to  the  first  material. 

The  sweat  glands  attain  a  large  size  in  certain  positions,  and  in  some 
of  these  larger  forms  the  secretion  is  probably  different.  One  kind  of 
modification  is  probably  the  very  large  wax  glands  found  in  the  ear  tube. 
In  structure  these  are  very  much  the  same  as  sweat  glands.  In  function 
they  secrete  and  discharge  a  thick  oily  material  that  almost  solidifies 
upon  contact  with  the  air  and  serves  as  a  protection  to  the  ear  tube 
against  the  entrance  of  harmful  insects  or  foreign  particles.  Figure 
363  shows  a  section  of  the  epi- 
thelium lining  the  coiled  por- 
tion of  this  wax  gland  with  the 
secretion  shown  in  situ  in  the 
lumen. 

See  also  the  description  of 
the  mammary  glands  in  Chap- 
ter XXIII,  as  they  are  also 
probably  derived  from  primitive 
sweat  glands. 

Technic.  —  Flemming's  fluid 
and  thin  paraffin  sections  will 
give  the  best  results  with  nearly 
all  of  these  tissues.  The  oily 
secretions  are  sometimes  stained  black,  and  sometimes  not,  by  the 
osmic  acid.  When  the  lubricating  secretion  is  a  mucin,  it  is  best  to 
use  sublimate  or  some  similar  fixative,  and  then  to  stain  with  one  of 
the  dyes  that  is  specific  for  this  substance.  Most  beautiful  differentia- 


FlG.  363.  — Transverse  section  through  one  of  the 
secretory  coils  of  a  wax  gland  in  the  cat's  ear- 
tube.  Some  secreted  material  lies  in  the  lumen. 


4OO  HISTOLOGY 

tions  may  often  be  obtained  in  this  way.  Mayer's  muci-carmine  is  a 
good  stain  for  this  purpose.  Delafield's  hseraatoxylin  is  perhaps  the 
best. 

LITERATURE 

HAMMARSTEN,  O.     "Studien  iiber  Mucin  und  mucinahnliche  Substanzen,"  Arch.  f.  d. 

ges.  Physiol.,  1885,  Band  XLVI,  S.  373. 
NOLL,  A.     "  Das  Verhalten  der  Driisengranula  bei  der  Sekretion  der  Schleimzelle  und  dei 

Bedeutung  der  Giannuzzi'schen  Halbmonde,"  Arch.  f.  Physiol.,  1902,  Suppl.  Band, 

S.  166. 
HAIDENHAIN,  M.     "tlber   die  Struktur  der  Darmepithelzellen,"  Arch.  f.   mik.   Anal., 

1899,  Band  LIV,  S.  184. 
HAGEN-TORN,  OSCAR.    "Entwicklung  und  Bau  der  Synovialmembranen,"  Arch.  f.  mik. 

Anal.,  Band  XXI,  1882. 
LOWENTHAL,  N.     "Zur  Kenntnis  der  Glandula  infraorbitalis  einiger  Saugetiere,"  Anal. 

Anz.,  1895,  Band  X,  S.  123. 
PFITSNER,  W.     "Das  Epithel  der  Conjunctiva,"  Zeitschr.  f.  Biol.,  1897,  N.  F.,  Band 

XVI,  397. 
PIERSOL,  G.  A.     "Beitrage  zur   Histologie  der  Harder'schen  Driisen  der  Amphibien," 

Arch.f.  mik.  Anat.,  1887,  Band  XXIX,  S.  594. 
GURWITSCH,  A.     "Die  Vorstufen  der  Flimmerzellen  und  ihre  Beziehungen  zu  Schleim- 

zellen,"  Anat.  Anz.,  1901,  Band  XIX,  S.  44. 


TISSUES  FOR  PRODUCING  ATTRACTIVE  AND  OFFENSIVE   ODORS 

In  the  mammals,  and  even  in  the  Sauropsida,  there  are  glands  of  the 
sebaceous  type  that  are  developed  and  specialized  to  perform  other  func- 
tions than  lubrication.  The  anal  glands  of  the  Camivora  are  sebaceous 
glands,  concentrated  in  number  and  developed  in  size.  Associated 
with  them  are  groups  of  saccular  glands  developed  from  the  basal 
layer  of  the  stratified  epithelium.  These  glands  are  not  very  well 
adapted  for  lubrication  purposes,  and  the  fact  that  they  are  more  or 
less  odorous  would  lead  one  to  believe  that  they  served  some  end  in 
the  life  and  habits  of  the  animal  by  giving  off  a  distinctive  scent.  This 
idea  is  supported  by  a  provable  fact  when  we  encounter  these  same 
glands  in  two  of  the  mammals,  the  muskrat  and  the  skunk.  Here  can 
be  recognized,  in  structure,  the  anal  glands  of  the  other  mammals,  enor- 
mously enlarged,  and  producing  a  secretion  that  in  the  one  case  may 
be  considered  attractive  and  in  the  other  is  very  offensive. 

In  section,  these  glands  present  but  slight  differences  to  the  eye  from 
the  common  sebaceous  glands  of  the  hair  in  the  other  mammals. 
They  are  somewhat  larger,  with  larger  cells  and  clearer  cytoplasm,  and 
are  placed  on  special  primary  invaginations  of  the  integument  on  each 
side  of  the  anus.  The  saccular  glands  with  simple  secreting  epithe- 
lium are  placed  in  groups  that  surround  the  glands  of  the  sebaceous 
type.  They  thus  are  collected  into  a  single  mass,  the  scent  glands 


ATTRACTIVE  AND  REPULSIVE    ODORS 


401 


(Fig.  364).  The  cells  of  the  sebaceous  glands  produce  an  oil  and  pro- 
duce it  in  the  same  way  that  the  ordinary  sebaceous  glands  produce  it, 
as  far  as  we  can  see.  But  the  chemistry  and  the  physiology  of  the 


b.  m. 

FIG.  364.  —  Axial  section  of  a  single  acinus  of  the  scent  gland  of  a  skunk,  Mephitis,  b.m.,  base- 
ment membrane;  x,  boundary  between  secreting  epithelium  and  stratified  epithelium  of 
upper  gland  and  duct;  dis.,  distal  surface  of  secreting  epithelium  where  it  becomes  the 
secretion.  X  650. 

process  must  be  different,  for  the  oil  produced  is  not  a  simple  lubricant, 
but  has  volatile  constituents  that  cause  a  most  powerful  odor.  The 
secretion  of  the  saccular  glands  is  watery  and  either  acts  merely  as  a 
carrier  for  the  odorous  oil  or  is  itself  a  constituent  part  of  the  scent- 


402 


HISTOLOGY 


producing  discharge.  Many  other  mammals  produce  fluids  by  very 
similar  structures,  which  are  attractive  or  repulsive.  The  musk  ox, 
bat,  etc.,  show  these  organs. 

The  Odoriferous  Glands  of  the  "Stink-pot"  Turtle.  —  Among  the 

reptiles  are  some  that  are  offensive  to  the  smell.    One  of  such  is  the  com- 

i;, .,..5;.  mon    stinking    turtle  of    the 

•;   :-  .  .         :•      :;-<;v.,..  eastern  United  States  which, 

when  captured  or  handled 
roughly,  gives  off  a  most 
disagreeable  and  offensive 
odor.  To  do  this  it  dis- 
charges drops  of  an  oily  fluid 
from  the  ducts  of  two  sym- 
metrically placed  glands  or 
sets  of  glands  that  are  placed 
just  inside  of  the  shell  on 
each  side  of  the  body.  These 
glands  are  developed  onto- 
genetically  by  invaginations 
of  the  integument  at  the 
point  where  the  duct  opens, 
and  they  are  evidently  lined 
by  a  stratified  epithelium  de- 
rived from  that  on  the  outer 
integument  of  the  animal  (Fig. 
365).  This  stratified  epithe- 
lium is  constantly  proliferat- 
ing  as  was  also  that  in  the 
rump  gland  in  the  bird,  and 
as  all  stratified  epithelia  are. 
It  is  also  secreting  an  oily 
fluid,  but  there  are  minor 
differences  in  the  manner  in 
which  the  two  kinds  of  glands 
do  this  as  well  as  in  the  product.  In  the  turtle  the  stratified  nature  of 
the  layer  is  obscured  by  the  fact  that  there  are  only  two  distinct  strata, 
an  inner  or  proximal  which  represents  the  basal  layer  of  a  stratified 
epithelium,  and  an  outer  or  distal  stratum  of  many  cells  in  thickness. 
The  single  basal  layer  is  constantly  dividing  off  cells  which  are  pushed 
into  the  thick,  distal  layer  of  cells  and  which  begin  to  accumulate  the 
secretion  as  soon  as  they  become  independent  of  the  basal  layer. 

The  secretion  appears  in  a  single  vacuole  on  the  distal  cytoplasm  of 
the  cell,  and  this  vacuole  enlarges  and  swells  the  cell  to  very  many  times 


6.J0..C 


FIG.  365.  —  Part  of  the  wall  of  the  stink  gland  of  a 
turtle,  Terrapene  odorata.  b.c.,  layer  of  basal  cells; 
o.c.,  oil  cells  lying  among  the  many  serous  cells; 
m.s.,  matured  serous  cell  with  dead  nuclei  and 
ready  to  burst;  b.o.c.,  young  oil  cells  on  the  base; 
sec.,  secretion.  X  650. 


ATTRACTIVE  AND  REPULSIVE    ODORS  403 

its  original  diameter.  The  nucleus  remains  round  and  full  and  does 
not  deteriorate  in  any  visible  degree.  When  fully  developed,  the  secre- 
tion is  in  the  form  of  closely  packed  granules,  and  the  cells  float  free  from 
the  epithelium  in  the  loose  secretion  that  occupies  the  lumen  of  the  gland. 
After  this  the  granules  dissolve  into  a  fluid  and  the  stroma  of  the  cell 
may  rupture,  setting  the  secretion  and  the  nucleus  free,  or  it  may  remain 
intact  for  a  long  period,  retaining  the  secretion  and  nucleus,  which  latter 
has  by  this  time  lost  its  internal  structure  while  yet  remaining  round 
and  full  in  outline.  When  the  secretion  is  discharged,  all  the  remaining 
cells  are  probably  ruptured,  setting  the  secretion  free. 

The  above  are  the  principal  cells  of  this  gland,  but  the  secretion  is 
contributed  to  by  another  set  of  cells  in  a  very  small  degree.  This  latter 
kind  is  found  at  rare  intervals  in  the  basal  layer  as  well  as  in  the  other 
layer  of  the  epithelium  of  the  gland  wall.  The  secretion  of  these  cells 
is  a  thick,  heavy,  yellow  oil  which  is  sometimes  placed  in  a  single  large 
globule,  but  more  often  appears  in  smaller  ones.  A  remarkable  feature 
is  that  more  of  these  cells  with  their  golden-yellow  contents  are  to  be  seen 
outside  of  the  gland  in  the  surrounding  connective  tissue  than  inside  the 
basement  membrane.  The  inside  cells  of  this  kind  have  more  probably 
been  basal  layer  cells  that  have  developed  the  oil-secreting  power  than 
connective-tissue  cells  that  have  moved  through  the  basement  mem- 
brane, carrying  their  load  of  secretion  with  them. 

These  glands  are  present  in  most  turtles,  although  not  in  all.  They 
are  either  lubricating  glands  or  scent  glands  in  all  the  cases  in  which  they 
occur,  but  are  undoubtedly  odorous  in  the  animal  in  which  we  have  stud- 
ied them.  Somewhat  analogous  glands  are  found  in  the  integument  of 
other  reptiles,  sometimes  on  very  different  parts  of  the  body,  as  the  glands 
on  the  jaw  of  .the  alligator  and  the  glands  on  the  thigh  of  the  lizard. 
Some  of  these  may  not  be  glands  for  producing  odor,  but  most  of  them 
probably  are.  The  secretion,  it  must  be  noted  in  this  case  as  well  as  in 
that  of  the  skunk,  is  not  discharged  automatically,  as  in  the  lubricating 
glands  of  the  mammals  (sebaceous  glands)  but  is  retained,  and  discharged 
as  needed. 

In  some  of  the  Amphibia  an  integumental  gland  is  used  to  produce 
an  offensive  fluid  (said  to  be  poisonous  to  some  of  its  enemies).  These 
glands  are  somewhat  like  the  surrounding  lubricating  or  mucous  glands 
in  the  same  animal's  integument,  and  consist  of  single  saccular  glands 
lined  by  a  simple  columnar  epithelium. 

The  so-called  poison  gland  of  Bufo  will  represent  this  class  of  tissue 
as  found  in  both  the  urodela  and  anura.  This  gland  is  vastly  larger  than 
the  mucous  glands  found  in  the  same  integument,  although  it  is  arranged 
on  much  the  same  plan.  A  portion  of  its  wall  is  represented  in  Figure 
366.  The  cells,  of  the  single  layer  of  epithelium  which  lines  it,  develop 


404 


HISTOLOGY 


sec.  c. 

I 


sec.  g 


;*JW£v 

?liii% 

im 
11 


a  huge  quantity  of  granular  material  which  destroys  the  distal  part  of 
their  cytoplasm  and  fills  the  entire  lumen  solidly  with  the    secretion. 

This  secretion  is  the   offen- 
sive  matter.    The  proximal 
parts  of  the  cells  remain  un- 
?••&*<&  differentiated,    and    form    a 

lining  of  cytoplasm  in 
which  the  flattened  nuclei 
lie. 

Outside  of  the  nuclear 
layer  appears  a  single,  close- 
set  layer  of  smooth  muscle 
cells,  two  of  which  are 
shown  in  oblique  section  in 
Figure  366.  They  serve  to 
contract  the  gland  when  the 
secretion  is  to  be  discharged. 
Covering  the  muscle  layer 
is  a  light  connective  tissue 
which  contains,  between  its 
thin  outer  and  inner  plates, 
a  plexus  of  capillaries.  Still 
outside  of  this  is  a  septum 
of  connective  tissue  which 
lies  between  all  of  these 
closely  placed  glands.  A 
duct  is  present,  in  a  more  specialized  form  than  in  the  neighboring 
mucous  glands.  It  has  a  complicated  set  of  muscles  for  controlling  the 
discharge  of  the  secretion.  When  the  gland  is  discharged,  its  lining  is 
destroyed  and  renewed  by  the  growth  of  a  bud  from  the  side,  as  has 
been  described  by  Esterly  and  others. 

The  secretion,  when  discharged,  is  a  modified  mucus,  and  is  watery 
instead  of  oily,  as  in  the  preceding  vertebrate  forms.  The  organs  have 
been  considered  as  offensively  odorous  rather  than  poisonous,  because 
the  animal  has  no  means  of  injecting  the  secretion  into  the  blood  of  an 
enemy  or  victim. 

Vastly  different  from  any  of  the  above  are  the  various  tissues  found  in 
invertebrate  animals,  and  which  are  used  to  produce  either  attractive  or 
repulsive  odors.  We  naturally  understand  those  found  in  the  air- 
breathing  animals  best.  There  are  undoubtedly  many  such  organs  to 
be  found  in  the  water-breathing  animals  that  we  do  not  know  of  and 
may  never  find. 

It  is  among  the  insects  that  the  greatest  number  and  variety  of  odorous 


FIG.  366. — Part  of  the  wall  of  a  repugnatorial  gland 
from  the  skin  of  a  toad,  Bufo.  sec.c.,  secreting  cells; 
sec.g.,  granular  secretion;  mus.c.,  parts  of  two  of  the 
investing  smooth  muscle  cells;  bl.l.,  connective-tissue 
layer  containing  the  blood  supply  in  capillaries; 
conn.t.,  connective- tissue  septum  belonging  to  the 
corium  and  found  between  all  the  glands.  X  1000. 


ATTRACTIVE  AND  REPULSIVE    ODORS 


405 


glands  are  found,  both  offensive  and  agreeable  to  their  enemies  and 
friends.  Some  of  these  are  external  integumentary  organs  and  others 
are  internal  glands  derived  from  the  integument.  We  shall  treat  of 
several  of  the  prominent  types  here.  The  great  variety  and  number  of 
these  organs  is  astonishing.  There  are  literally  thousands  of  insects 
that  possess  the  structures  in  many  positions  on  the  body.  In  some  they 
are  temporary  larval  structures.  In  most  cases,  as  far  as  is  known, 
they  are  either  integumentary  invaginations  or  they  are  accessory  anal 
or  oral  glands. 

A  fair  type  of  an  integumentary  gland  producing  an  ill-smelling  fluid 
is  to  be  found  in  the  earwigs  or  Forficulidae.  The  opening  of  these  glands, 
which  are  found  at  points  on  the  latero-dorsal  surface  of  the  posterior 
part  of  the  abdomen,  gives  grounds  for  thinking  that  it  is  an  integumen- 
tary imagination. 

Dissection  shows  that  the  gland  is  shaped  somewhat  like  a  retort,  or 
is  "pear-shaped."  The  narrow,  funnel-shaped  neck  opens  on  the  ex- 
terior by  a  very  tiny  ringed  opening  in  the  chitin.  The  innermost  layer 
is  a  thin  chitinous  membrane  that  is  striated  irregularly  and  which  gives 
off  plate-like  and  tube-like  processes  proximally  into  the  underlying 
(outer)  cellular  layer.  This  layer  is  composed  of  cells  which  are  directly 
continuous  with  the  hypodermal  cells  of  the  outer  integument,  as  also 
the  inner  chitinous  layer  is  continuous  with  the  cuticle  of  the  body. 

The  next  point  is  to  see  how  the  gland-cells  are  placed  and  how  the 
secretion  is  freed.  The  cellular  cover- 
ing is  composed  of  two  kinds  of  cells 
(Fig.  367).  The  smaller  are  of  no  par- 
ticular interest,  and  have  small  oval 
nuclei  and  transparent  bodies  whose 
boundaries  are  hard  to  see.  The  plate- 
like  processes  of  chitin  extend  into  the 
cytoplasm  in  places.  The  second  sort 
of  cells  are  very  large  and  of  great 
specialization  and  complexity.  The  nu- 
cleus is  extremely  eccentric  and  flat- 
tened somewhat.  It  is  surrounded  by 
a  thin,  clear  space.  The  larger  part  of 
the  cell  body  is  occupied  by  a  clear  cyto- 
plasm which  contains  and  probably  also 
secretes  the  ill-smelling  fluid.  The  fluid 
is  drained  from  the  cell  by  one  of  the 
chitinous  tube-shaped  invaginations  of 
the  inner  cuticle.  This  fine  capillary  lies  with  its  end  coiled  up  in  the 
clear  area  of  the  gland-cell. 


7  sec.r. 


FIG.  367.  —  Two  secreting  cells  from 
the  stink  gland  of  the  earwig,  sec.r., 
secretion  region  of  the  cytoplasm; 
t.,  parts  of  tubule  by  which  the  secre- 
tion is  withdrawn  from  the  cell. 
(After  VOSSELER.)  ' 


406 


HISTOLOGY 


There  are  not  many  of  these  large  gland-cells.  They  are  scattered 
singly  or  in  groups  through  the  membrane,  and  their  combined  secretions 
fill  the  flask-shaped  gland  sac.  A  muscle  fiber  holds  the  neck  and  regu- 
lates the  discharge  of  the  fluid.  The  secretion  is  a  most  pungent  and 
offensive  fluid.  It  is  acrid  and  volatile. 

The  Hemiptera  have  in  nearly  all  cases  a  pair  of  glands  in  the  body 
which  open  as  the  earwig's  did  on  the  surface.  The  surface,  however, 
is  on  the  ventral  side  near  the  third  pair  of  legs.  These  glands  secrete 
a  fluid  which,  while  widely  different  in  the  many  species,  has  a  peculiar 
odor  characteristic  of  all  true  bugs.  In  some  cases  it  is  intensely  offen- 
sive, and  in  others  it  is  actually  fragrant  or  pleasing,  having  a  "fruity" 
odor.  The  glands  are  of  various  shades  of  red  and  brown  and  yellow 
and  represent  a  distinct  type.  We  shall  examine  these  scent  glands 
from  a  large  species,  Belostoma  Americana,  in  which  they  form  two 
symmetrical,  elongated  sacs  or  tubes.  The  color  is  a  very  light  yellow, 
and  while  the  animal  seldom  uses  them,  when  one  is  dissected  and  the 
tissue  is  cut,  the  odor  is  intense. 

Sections  show  that  this  gland  like  that  of  the  earwig  is  an  invagi- 
nation  of  the  integument  (Fig.  368).  It  is  a  far  more  involved  gland, 
being  an  irregularly  tubular  gland,  convoluted  in  some  regions.  Its 
walls  are  thick  and  lined  on  the  outside  by  a  very  delicate  connec- 
tive tissue  which  holds  the  numerous  tracheae  in  contact  with  it. 

The  important 
layers  of  its  walls 
are  so  involved 
that  it  is  .hard  to 
describe  any  one 
without  also  de- 
scribing the  others. 
The  innermost  is 
a  very  much  crum- 
pled layer  of  chitin 
continuous  with 
the  exterior  cuticle 
through  the  neck 
of  the  gland.  At 
numerous  points 
which  are  usually 
situated  on  the 
proximal  flexures 
of  the  layer,  are 
tiny  openings  into  which  the  walls  of  the  layer  are  produced  downward 
as  a  fine,  clearly  marked  tubule  of  even  caliber  and  slightly  curving 


_- - •d. 


FIG.  368.  —  Bit  of  epithelium  from  the  odorous  gland  of  the  Hemip- 
ter,  Belostoma.  cu.,  cuticle;  hyp.c.,  hypodermal  cells  which  se- 
crete the  cuticle;  sec.c.,  secreting  cells,  each  with  a  large  collecting 
vacuole,  vac.;  s.s.,  secretion  seive;  d.,  duct  to  carry  secretion  to 
the  lumen;  sec.,  secretion  emerging  from  duct.  One  seive  and  its 
duct  is  shown  torn  out  and  separated  from  the  cell  to  which  it 
was  attached.  X  1275. 


ATTRACTIVE  AND  REPULSIVE   ODORS  407 

course.  Besides  these  tubules,  some  plates  of  what  appears  to  be  chitin 
pass  down  between  some  of  the  cells. 

The  next  two  layers  are  cellular.  That  lying  next  to  the  cuticle  is 
very  thin  and  appears  as  almost  a  line  except  where  its  contained  nuclei 
give  it  a  greater  breadth.  It  follows  the  sinuosities  of  the  cuticle  and  its 
nuclei  are  placed  most  often  in  the  upper  curvatures.  The  tubules  just 
spoken  of  penetrate  this  layer  through  or  between  its  cells.  It  seems 
more  probable  that  they  pass  through  these  cells. 

Immediately  beneath  this  layer  is  found  the  layer  of  secreting  cells. 
They  are  thick  and  heavy,  being  roughly  twelve  to  fifteen  times  the  thick- 
ness of  the  chitinous  layer  and  five  to  seven  times  its  thickness  in  width. 
Some  of  the  narrower  ones  are  not  secreting,  and  possibly  may  develop 
later  into  gland-cells.  Those  that  are  secretory  show  a  very  large,  clear 
vacuole  slightly  above  their  middle  height.  This  vacuole  is  marked 
off  from  the  heavy,  granular  cytoplasm  by  a  distinct  membrane.  It  is 
evidently  homologous  with  the  clear  cytoplasmic  space  described  in  the 
odoriferous  cell  in  Forficula,  but  is  a  higher  specialization.  That  in 
Forficula  had  no  bounding  membrane  besides  other  differences  to  be 
noticed  in  the  drawings  and  descriptions. 

The  cuticular  tube,  mentioned  above  as  coming  down  through  the 
hypodermal  cells  from  the  cuticle,  enters  into  the  top  of  a  secretory  cell 
and  penetrates  to  the  vacuole  which  it  enters.  Here  it  ends  in  a  round 
knob  and  it  also  apparently  ends  blindly,  for  its  lumen  comes  to  a  blind 
point  in  the  center  of  the  knob,  and  there  is  apparently  no  further 
opening. 

Fine  fibrils  reach  from  all  points  of  the  vacuolar  wall  to  this  knob 
which  is  placed  near  its  distal  wall.  They  make  a  beautiful  radial  pic- 
ture which  is  well  seen  in  three  places  in  Figure  368. 

The  cuticular  tube  obviously  serves  the  purpose  of  conducting  away 
the  secretion.  The  fact  that  a  simple  tube  can  and  does  perform  this 
function  in  the  earwig  adds  to  the  mystery  of  why  a  round,  heavy 
knob  should  be  placed  over  the  end  of  the  same  structure  in  Belostoma. 
That  it  does  carry  out  the  secretion  in  this  animal  seems  more  than  in- 
dicated by  the  collected  granular  material  heaped  up  in  the  cuticle  flexure 
into  which  it  opens. 

A  word  must  be  said  here  regarding  the  origin  of  the  secretory  layer 
in  this  gland.  If  the  thin,  hypodermal  layer  directly  under  the  cuticle 
represents  a  perfect  layer  and  all  of  the  ectodermal  cells  in  the  gland, 
then  the  secretory  cells  are  mesodermal  in  origin  and  the  penetration  of 
then-  body  by  the  cuticular  tubule  is  a  secondary  relation.  On  the  other 
hand,  the  presence  of  this  tubule  and  the  fact  that  the  cells  are  secretory 
lead  the  writers  to  believe  that  the  secretory  cells  are  derived  in  the 
embryo  from  the  hypodermal  layer  and  have  acquired  their  proximal 


4o8 


HISTOLOGY 


position  by  moving  down  out  of  this  layer  as  do  the  acid  cells  from  the 
epithelium  of  the  gastric  glands  of  the  stomach  of  mammals. 

The  Myriapoda  have  odorous  glands  which  produce  hydrocyanic 
acid  gas.  Julus  is  the  best  example  and  the  gland  is  exceedingly  simple. 
It  is  again  an  integumental  invagination,  a  simple,  saccular  form,  the 
color  of  whose  tissues  gives  it  a  dark,  purple  shade.  Its  duct  possesses 
a  most  interesting  closing  device,  as  we  must  call  it,  because  its  elasticity 
causes  it  to  remain  closed  until  the  muscle  fiber  opens  it  (Fig.  369). 
The  wall  of  the  duct  is  made  of  an  inner  chitinous  layer  and  an  outer 
cellular  layer,  the  hypodermis.  Near  its  exit,  one  side  of  this  tube  is 
evaginated  into  the  lumen  of  the  tube,  and  the  lower  end  of  this  process 
is  forced  down  into  the  external  opening.  This  is  its  natural  position, 


B 


FIG.  369. — Three  sketches  to  illustrate  the  apparatus  by  which  the  discharge  of  prussic  acid 
from  its  repugnatorial  glands  is  controlled  by  Julus.  A ,  general  side  view  with  inside  out- 
lines indicated  by  dotted  lines;  stopper  in  place.  B,  slightly  oblique  section  to  show  outer 
opening  of  duct  and  the  stopper  withdrawn.  C,  transverse  section  to  show  relations  of 
muscle  and  stopper.  Lettering  for  all  figures :  m.,  mouth  or  outer  opening  of  tube;  5. .stop- 
per; mus.,  muscle  by  whose  contraction  the  stopper  is  withdrawn;  cu.,  outer  surface  of  cu- 
ticle, seen  from  the  surface  in  A  and  from  the  side,  in  section,  in  B  and  C.  (After  Rossi.) 

and  so  the  tube  is  closed  and  stoppered  when  at  rest.  A  muscle  fiber 
from  some  point  of  attachment  on  the  ring  segment  enters  the  hollow 
lumen  of  this  evagination,  and  when  it  pulls,  the  stopper  is  withdrawn, 
and  the  tube  acquires  a  horseshoe-shaped  lumen  which  permits  the 
secretion  to  escape. 

Beneath  the  cuticular  lining  of  the  sac  lies  the  secreting  epithelium. 
Unlike  that  of  the  two  insect  examples  we  have  been  studying,  this  is 
a  simple,  cuboidal  epithelium.  It  secretes  the  poisonous  and  offensive 
hydrocyanic  acid  which  one  can  smell  when  handling  the  creature. 

Many  insects  give  off  an  odor  that  is  not  only  agreeable  to  other  in- 
sects but  to  man  as  well.  They  are  distinctly  alluring  glands.  These 
glands  are  found  in  still  more  peculiar  places  in  the  anatomy  of  the  crea- 
tures than  even  the  offensive  glands  were.  A  favorite  position  in  the 


ADHESION  AND   SPINNING  409 

butterflies  seems  to  be  in  various  sacs  on  the  body  or  in  various  positions 
on  the  wings.  In  these  cases  the  hypodermal  cells  which  formed  the 
scales  or  hairs  also  secrete  a  fluid  which  is  discharged  through  the  scale 
in  minute  quantities,  and  vaporizes  in  the  atmosphere. 

While  many  invertebrates  below  the  insects  have  peculiar  odors  we 
cannot  discuss  them  as  special  odorous  organs  because  enough  is  not 
known  about  them.  This  is  especially  true  of  animals  living  in  the  water, 
although  it  is  satisfactorily  proven  that  some  forms  of  water  animals 
have  a  keen  sense  of  smell  (selachian  fishes). 

Technic.  — These  tissues  are  usually  fairly  easy  to  cut,  and  paraffin 
section  from  Flemming  and  Zenker  material  show  all  that  is  to  be  seen 
with  the  exception  of  the  innervation  and,  perhaps,  blood  distribution. 


LITERATURE 

Rossi,  G.     "Le  glandole  odorifere   dell'  Julus  communis,"  Zeits.  f.  wiss.  Zool.,  Band 

LXXIV,  S.  64,  1903. 
WEBER.      "Uber  eine   Cyanwasserstoffsaur    bereitende     Druse,"   Arch.  f.   mik.    Anat., 

Band  XXI,  S.  468,  1882. 
ZIETZSCHMANN,  H.     "  Beit  rage  zur  Morphologic  und  Histologie  einiger  Hautorgane  der 

Cerviden,"  Zeits.  f.  wiss.  Zool.,  Band  LXXIV,  S.  i,  1903. 
WILLISTON,  S.  A.     "A  Protective  Secretion  of  Eleodes  ejected  from  the  Anal  Gland," 

Psyche,  Vol.  IV,  p.  168,  1884. 

BORGERT,  H.     "Die  hautdrusen  der  Tracheaten,"  In.  Diss.,  Jena,  1891. 
VOSSELER,  J.     "Die  Stinkdrusen  der  Forficuliden,"  Arch.f.  mik.  Anat.,  Band  XXXVI, 

S.  565,  1890. 


TISSUES   OF  ADHESION   AND   SPINNING 

These  structures  are  found  in  a  number  of  lower  animals  where  they 
are  fastened  to  a  rock  or  other  surface.  They  form  a  fairly  homogeneous 
group  of  tissues  and  even  the  organs  that  these  tissues  compose  afford 
a  number  of  close  homologies.  The  two  principles  involved  in  most 
of  them  are  the  development  in  a  columnar  epithelium  of  two  features; 
a  set  of  non-elastic  fibrils  to  afford  strength  and  points  to  attach,  and 
the  secretion  of  some  tough,  gummy  substance  to  cause  the  cells  to  adhere. 
In  larger  forms,  epithelial  cells,  which  are  usually  invaginated  into 
glands,  are  used  to  secrete  an  adhesive  fluid  that  can  be  extruded  as  a 
thread  which  sticks  and  hardens.  In  other  forms  the  principle  of  suc- 
tion is  brought  into  use  and  various  suckers,  pads,  etc.,  are  used  to  attach 
the  creature,  usually  temporarily,  to  some  surface.  Mechanical  grasping 
organs  will  not  be  considered  here. 

The  Protozoa  show  many  forms  of  this  power  which,  unfortunately, 
cannot  be  properly  studied  on  account  of  the  weak  development  and 


4io 


HISTOLOGY 


-ad.sec. 


6.  m.. 

FIG.  370. — Three  epidermal  cells 
from  the  proximal  surface  of  the 
"foot"  of  Hydra,  b.m.,  basement 
membrane;  ad.sec.,  granules  of 
adhesive  secretion  on  the  distal 
parts  of  the  threads.  X  870. 


specialization  that  is  necessary  in  so 
small  animals.  The  attachment  is  either 
temporary  or  permanent. 

The  Coelenterates  are  rich  in  the  num- 
ber of  forms  that  attach  themselves  by 
some  part  of  their  surface.  Hydra  will 
show  an  evident  example  of  a  typical 
form  of  adhesive  structure.  Sections  of 
the  epithelium  on  the  "foot"  show  that 
this  tissue  has  become  thicker  and  stronger 
by  the  development  of  this  power  (Fig. 
370).  On  closer  examination  one  can  see  that  the  individual  cells  have 
well  developed  fibrils  in  their  cytoplasm  reaching  from  proximal  to 
distal  surface  and  better  developed  distally  than 
proximally.  This  is  a  contrast  with  the  few 
weak  fibrils  found  in  other  epithelial  cells  on 
the  body  because  these  latter  are  far  more  ir- 
regular in  direction  and  thus  not  so  well  fitted 
mechanically  to  bear  a  strain.  The  next  point 
to  be  noticed  is  that  in  the  outer  fifth  of  the 
cytoplasmic  body  of  the  cell,  a  number  of  se- 
cretion granules  appear  lying  against  the  fibrils. 
These  are  undoubtedly  secretion  granules  of  the 
adhesive  material  that  is  used  to  make  the  foot 
stick  to  the  surface  on  which  the  animal  rests. 

Other  surfaces  on  the  various  Cnidaria  have  a  similar  structure,  some- 
times much  specialized,  but  developed  on  the  same  principle.     We  shall 

go  to  the  Ctenophora  and  ex- 
amine a  specimen  of  epithelium 
that  is  used,  not  to  hold  the 
body  in  any  particular  position, 
but  to  seize  the  prey  upon 
which  the  animal  feeds. 

The  tentacular  epithelium 
of  Beroe  ovata  has  been  de- 
scribed by  Schneider,  and 
shows  this  seizing  apparatus 
splendidly  developed  (Figs.  371, 
372,  and  373).  The  apparatus 

FIG.  372.  —  Half-developed  group  of  grasping  cells      .  imirpllnlar    hnt    rnndctc 

from  Beroe  ovata.     n.,  nucleus  of  grasping  cell;      1S    nOt    Unicellular    DUt    COnSlStS 
nx.c.,  nucleus  of  cap  cell;  n.c.t.,  nucleus  of  con-      of  a  number  of  groups  of  Cells, 


FlG.  371.  —  Very  young  stage 
of  a  group  of  grasping  cells 
from  Beroe  ovata.  g.c.,  two 
young  grasping  cells ;  sec.g., 
secretion  granules;  c.c., 
single  cap  cell.  (After 
SCHNEIDER.) 


mucin  ceils.   (After  SCHNEIDER.) 


three  to  seven  cells  extending 


ADHESION  AND  SPINNING 


from  the  mesogloeal  line  as  fibers,  almost  to  the  surface.  Straight  and 
thread-like  in  their  proximal  course,  they  arise  and  become  ribbon-like 
when  halfway  up.  This  ribbon-like  part  is  wound  in  a  spiral  form 
around  the  nucleus. 

Lying  as  a  cap  over  the  nucleus  and  the  fibrillar  portion  is  the  glue- 
forming  cytoplasm  with  the  vacuoles  of  adhesive  substance  covering  its 
outer  surface.  Just  under  these 
vacuoles  lies  a  layer  of  large, 
round  plastid-areas  that  represent 
some  secretory  activity.  They 
have  been  suggested  to  be  poison- 
forming  areas  of  the  cytoplasm, 
but  they  more  probably  represent 
preparatory  stages  in  the  forma- 
tion of  the  glue  substance.  A 
very  peculiar  cap-like  mass  on  the 
distal  end  of  the  oval  nucleus  is 
not  as  yet  understood,  and  we  can 
assign  no  function  to  it. 

A  few  other  cells  are  found  in 
connection  with  these.  Some  epi- 
thelial cells  are  grouped  about  the 
bases  of  the  adhesive  cells  and 
help  support  them.  Outside  of 
them  is  a  layer  of  covering  cells 
and  during  their  growth  a  single 
"cap  cell"  covers  the  six  or  seven 
cells  in  the  group  and  protects 
them  until  "ripe."  A  few  slime 
cells  are  found  near  and  among 
them. 

Passing  from  the  Ccelenterata  to  the  Mollusca  a  somewhat  new  type 
of  apparatus  for  attachment  is  found  in  which  the  second  steps  in  speciali- 
zation have  taken  place,  the  invagination  of  the  glue-forming  cells  and 
the  formation  of  this  adhesive  substance  into  strands  used  to  attach  the 
animal  to  its  living  place.  This  apparatus  is  known  as  a  byssus. 

Here  it  is  the  "foot"  of  the  animal  that  develops  the  organ.  This  is 
not  because  the  "foot"  of  the  mollusk  is  homologous  to  that  of  the  ccelen- 
terate  but  because  both  of  them  are  used  to  rest  on  and  for  attachment. 

The  common  mussel,  Mytilus,  will  provide  a  subject  in  which  we 
may  see  a  well-developed  byssus.  This  should  be  dissected  out  with  a 
scalpel,  and  also  studied  in  a  series  of  bulk-stained  sections  for  a  general 
idea  of  the  apparatus.  When  it  is  desired  to  understand  the  histological 


FIG- 373-  —  Matured  grasping  cells  of  Beroe 
ovata.  sp.f.,  spiral  fiber  which  extends  from 
the  basement  membrane  (b.m.)  and,  ascend- 
ing to  the  main  cell  body,  winds  in  two  turns 
about  the  nucleus;  x,  centrosome-like  body 
on  distal  end  of  nucleus.  Other  letters  indi- 
cate same  as  in  the  preceding  figure.  (After 
SCHNEIDER.) 


412 


HISTOLOGY 


structure,  it  should  be  examined  in  a  form  in  which  the  organ  is  not  so 

highly  developed,  and  where  the  cellular  relations  are  more  easily  made 

out. 

The  byssus  gland  of  the  mollusk,  ^Enigma  cenigmatica,  as  described 

by  G.  C.  Bourne,  is  a  very  favorable  object  and  Figure  374  shows  part 

of  a  section  through  the  byssus 
gland  that  shows  the  most  impor- 
tant features.  This  structure  is  a 
deeply  invaginated  gland,  extend- 
ing in  from  the  surface  through 
a  duct,  which  widens  inside  into 
a  chamber  of  some  size.  This 
chamber  is  partly  divided  by  a 
median  septum  into  two  lateral 
halves,  and  from  the  upper  walls 
a  series  of  thin  lamellae  hang 
down,  forming  a  laminated  gland. 
The  secretion  is  produced  by  at 
least  two  sets  of  cells.  The  first 
part  appears  as  a  product  of  cells 
situated  in  the  deep  upper  parts 
of  the  lamellar  acini.  This  mate- 
rial is  moved,  as  a  thin  sheet  or 
lamina  of  the  peculiar  hyaline  bys- 
sus substance,  down  towards  the 
edges  of  the  lamella  or  folds  of  tis- 
sue bearing  the  cells  that  produce 
it  (Fig.  374,  sec.c.). 

The  middle  zone  of  these  la- 
bears  flat  cells  whose  sur- 


sec.c. 


la. 


FlG.  374-  -Vertical  section   through  the  edge 

and  upper  part  of  one  of  the  lamellae  of  the     face  is   beset  with  short,  Stiff  Cilia. 


byssus  gland  of  ^Enigma,     la.,  lamina  of  se- 
cretion  at  sec.;  cil.c.,  ciliated  cells  lining  the 


o  •,  -,  •,       •    j      ,1      , 

S°me     Writers     have     denied     that 


middle   region   of  the  sides   of  the  lamellae;     these  cilia  moved,  and  Others  have 


sec.c.,  upper  or  distal  secreting  cells. 
G.  C.  BOURNE.) 


(After   eyen    disciaimecj 


idea  that 

they  were  cilia.  The  writers  be- 
lieve them  to  be  short,  strong  cilia  which  move  the  lamina  of  material 
downward.  These  cells  secrete  no  material  (Fig.  374,  cil.c.). 

When  the  laminae  reach  the  distal  edges  of  the  lamellae  they  are  added 
to  by  the  epithelial  cells  of  this  edge  which  are  enlarged  and  crowded 
full  of  "  bissageneous  "  granules.  Figure  374  shows  this  condition  well. 
The  various  laminae  now  fuse,  in  some  cases  remaining  single,  and  form 
the  well-known  threads  which  attach  the  mussel  to  the  rocks. 

In  other  mollusks,  as  Anomia,  the  byssus  substance  is  formed  by 


ADHESION  AND  SPINNING 


413 


a  shallow  gland  as  a  calcified  shell-like  structure  which  is  fastened  per- 
manently to  the  rock. 

The  sucking  disk  of  the  fish-leech,  Pontobdella,  while  used  as  a  me- 
chanical sucker,  is  also  to  some  extent  an  adhesive  instrument.  The 
epithelium  that  covers  the  applied  surface  of  this  organ  is  stratified,  and 
the  small  amount  of  adhesive  material  is  more  of  a  mucus  than  a  glue. 


s.ep. 


FIG.  375.  —  Axial  section  through  the  grasping  disk  on  the  arm  of  a  cephalopod  mollusk;  the 
squid,  Loligo  Pealii.  s.ep.,  simple  epithelium  becoming  thickened  toward  rim;  sec.ep.,  se- 
cretory portion  of  simple  epithelium,  found  on  inner  sides  of  cup  to  secrete  the  stiff  cylinder 
(cy.)  which  is  here  distorted  in  the  process  of  preparation;  p.ep.,  epithelium  which  acts  as 
a  portion  or  plunger  to  produce  a  vacuum;  mus.,  muscle  which  operates  the  piston;  ped., 
muscular  pedicel  (in  outline)  by  which  the  organ  is  attached  to  the  arm.  X  600. 

Several  species  of  Aphrodite,  another  annelid,  produce  very  large 
quantities  of  an  adhesive  material  which  is  used  to  build  the  tubes  in 
which  the  animal  lives.  This  material  is  secreted  by  epithelial  cells 
which  are  invaginated  into  glands.  These  glands  correspond  to  the 
upper  setigerous  glands  which,  in  other  worms,  are  used  to  produce  the 
solid  setae. 


414  HISTOLOGY 

Another  organ  of  attachment  is  to  be  found  in  the  cephalopod  mol- 
lusks.  These  are  the  ''suckers"  which  are  found  on  the  arms  of  the 
various  species  of  cuttles  and  octopus.  They  are  outgrowths  of  the  arm 
on  its  inner  side,  and  each  one  consists  of  a  cup-shaped  organ  of  muscle 
and  connective  tissue,  covered  with  an  epithelium  which  is  here  simple, 
and  attached  to  the  arm  by  a  strong,  muscular  stalk  of  pedicle  (Fig. 

375)- 

There  are  three  regions  of  surface,  each  covered  with  a  different  kind 
of  epithelium  on  the  sucker-cup.  The  outside  is  covered  with  a  simple 
form  that  is  very  thin  on  the  proximal  part  of  the  outside,  and  which 
becomes  thicker  farther  down  on  the  outside,  until  it  is  thickest  at  the 
rim.  On  the  broad  edge  it  becomes  very  thick,  and  secretes  a  heavy, 
cuticular  ring.  This  secretion  is  continued  on  the  inner  sides  of  the  cup. 

The  bottom  of  the  cup  is  covered  with  simple  columnar  cells  that 
secrete  mucus.  This  bottom  is  movable  up  and  down  in  the  cylindrical 
cavity  of  the  cup  which  is  kept  rigid  and  open  by  the  thick  cuticle  on  its 
sides  and  edge.  The  muscle  fibers  which  operate  the  epithelial  "plun- 
ger" can  be  seen  as  parallel  fibers  passing  from  the  outer  surface  of  the 
bottom  across  to  the  inner  surfaces  of  the  plunger.  A  very  narrow  layer 
of  muscle  across  the  top,  with  its  fibers  lying  in  all  diameters,  serves  by 
its  contractions  to  compress  laterally  and  thus  to  elongate  the  plunger. 

When  elongated,  the  whole  lower  surface  of  the  cup  is  applied  to 
any  surface,  and  then  the  plunger  muscle  is  contracted.  This  acts  to 
produce  a  vacuum  in  the  cavity,  and  to  make  the  sucker  adhere  strongly. 
Weak  circular  muscle  fibers  are  found  in  the  upper  part  of  the  cup's 
sides.  The  figure  is  taken  a  little  to  one  side  of  the  stalk  which,  therefore, 
does  not  appear,  with  its  core-like  continuation,  in  the  center  of  the  suc- 
tion muscle,  but  is  indicated  in  outline. 

A  very  remarkable  organ  of  adhesion  is  placed  on  the  upper  side  of 
the  head  in  the  fish,  Remora.  This  sucker  acts  much  as  the  squid's 
sucker  did,  but  is  more  complex  in  structure.  It  consists  of  an  oval, 
pad-like  area  on  the  top  of  the  head.  The  integumental  edges  of  this 
pad  are  raised  to  form  a  rim,  and  somewhat  similar  folds  run  across  its 
least  diameter  in  parallel  lines.  Figure  376  shows  part  of  a  longitudinal 
section  of  the  organ,  showing  one  of  the  ridges.  All  folds,  both  the  outer 
edge  and  the  transverse  folds  are  covered  with  the  stratified  epithelium 
characteristic  of  fish  integument.  This  is  the  thickest  on  the  edges  of 
the  pad  and  on  the  tops  of  the  posteriorly  slanting  folds.  It  is  exceedingly 
thin  and  weak  in  the  hollows  and  on  the  under  sides  of  the  folds.  It 
can  thus  be  seen  that  the  organ  does  not  depend  upon  any  adhesive 
secretion,  and  an  examination  of  its  mode  of  operation  and  muscular 
structure  shows  that  it  depends  altogether  upon  suction. 

The  bed  of  the  pad  is  a  thick  connective  tissue  in  which  lies  a  thin 


ADHESION  AND  SPINNING  415 

plate  of  tough,  rigid  material,  homologous  in  its  structure  and  mode 
of  origin  to  the  fish  scale.  The  folds  each  have  for  a  central  body  a 
ridge  of  connective  tissue,  in  the  center  of  which  are  wide,  spine-like 
projections  that  arise  by  a  joint  from  the  bed-plate  and  extend  almost 
to  the  edge  of  the  fold.  Here  they  turn  toward  the  upper  surface  of  the 
fold,  and  frequently  cut  through  the  epithelium  and  project  freely.  The 
bed-plate  consists  of  a  number  of  jointed  parts,  and  is  backed  by  a  mus- 
cular mass  on  the  skull.  Different  branches  of  these  muscles  operate 
to  raise  the  ridges  and  elevate  the  edge  of  the  whole  pad.  The  Remora 


—spn. 


FIG.  376.  —  Longitudinal,  vertical  section  through  a  small  region  of  the  grasping  organ  on  the 
head  of  a  teleost  fish,  Remora.  One  of  the  plates  is  shown  and  can  be  seen  to  be  a  modified 
fin  ray.  spn.,  jointed  spine;  car.,  cartilaginous  base  of  organ.  X  80. 

applies  its  pad  to  the  side  of  a  shark,  a  swordfish,  or  a  whale,  and  the 
muscles  cause  a  rather  weak  suction  to  take  place.  The  greatest  suc- 
tion occurs  when  the  fish  is  drawn  backward,  as  by  the  motion  of  its 
host,  or  otherwise.  The  projecting  plates  then  tend  to  rise  and  thus 
create  a  vacuum  under  the  pad  which  is  made  to  adhere  the  firmer. 

The  feet  of  the  insects  show  some  notable  examples  of  temporary 
adhesion  by  means  of  a  gummy  secretion.  In  the  pine  weevil,  Hylobius, 
the  entire  sole  of  the  foot  is  crowded  by  a  great  mass  of  the  so-called 
"tenent  hairs"  which  are  single  hypodermal  cells  whose  bodies  remain 
in  the  foot,  and  whose  distal  ends  are  produced  into  long,  soft-walled  pro- 


416 


HISTOLOGY 


jections  which  secrete  a  viscid  fluid  that  causes  them  to  adhere  to  what- 
ever they  touch  (Fig.  377). 
These  hairs  are  less  abun- 
dantly developed  in  many 
other  insects,  sometimes  to  be 
used  only  during  copulation 
or  for  other  special  pur- 
poses. In  many  larvae  the 
gland  hairs  are  supple- 
mented by  various  spikes 
and  hooks,  all  formed  from 
hypodermis  cells  and  cuti- 
cle, as  all  other  external 
arthropod  tissues  and  organs 
are. 

FIG.   377.  — Adhesive  structure  on  foot  of    a  beetle,  o  ,     ,  ,.    ,  , 

Hylobius.    Transverse  section  of  foot,    ad.c.,  adhesive  ^ne    of    tne    most    highly 

cells  whose  processes  (ad.p.)  extend  downward.    (After     specialized       types      of      the 

organs     of     adhesion     and 

spinning  may  be  seen  in  the  insects  as  a  spinning  gland  of  various 
larvae  and  adult 
forms.  These  or- 
gans are  invaginated 
portions  of  the 
mouth  lining,  and 
are  well  represented 
by  the  spinning 
gland  of  the  larva 
of  a  moth,  Imperialis. 
This  gland  has  as- 
sumed the  form  of  a 
narrow  tube  of  some 
length  and  a  portion 
of  the  wall  of  this 
tube  is  shown  in  Fig- 
ure 378. 

The  cells  are  large 
and  cuboidal.  They 
form  a  single  layer 
lying  on  a  fairly  thick 
basement  membrane. 

The  cells  are  so 
large  that  they  are 
supplied  internally  with  tracheal  branches.  One  of  these  appears  in 


FIG.  378.  —  Portion  of  a  longitudinal,  vertical  section  through  the 
epithelium  of  a  silk  gland  in  the  larva  of  the  moth,  Imperialis. 
Three  large,  irregular  nuclei  (».),  representing  two  cells,  are 
shown,  tr.ca.,  tracheal  capillary  with  a  single  nucleus,  tr.  n.,  in 
its  wall  (modified  hypodermal  nucleus) ;  d.,  distal  edge  of  epi- 
thelium; b.m.,  basement  membrane;  sec.gr.,  secretion  granules. 


ADHESION  AND   SPINNING  417 

the  section  in  transverse  section,  showing  its  surrounding  covering  of 
cells  with  a  nucleus  belonging  to  one  of  them.  As  may  be  seen  in 
the  figure  the  lines  of  demarcation  of  the  cells  are  obscure  or,  in 
places,  not  perceivable.  A  marked  peculiarity  of  all  of  them,  in  the 
insects,  is  the  great  branching  of  the  single  nucleus  in  each  cell.  The 
figure  shows  three  sections  of  these  branches,  two  of  which  belong  to  a 
single  nucleus.  The  chromatin  is  finely  granular,  and  a  plasmosome 
does  not  appear  in  the  section. 

The  cytoplasm  is  granular,  and  some  of  the  granules  are  arranged 
in  larger  bodies  much  as  they  are  in  nerve-cells.  These  masses  have 
an  apparent  axis  which  shows  a  tendency  to  lie  parallel  with  the  main 
axis  of  the  cell.  They  are  also  larger  in  the  proximal  part  of  the  cell 
and  diminish  in  size  towards  the  distal  part.  The  distal  surface  of  the 
cell,  where  it  borders  on  the  lumen,  is  coarsely  but  weakly  striated.  The 
nuclei  of  these  cells  should  be  examined  in  a  freshly  dissected  gland  from 
a  cabbage  worm  (Pier is]  to  observe  their  peculiar  shape. 

The  silk  lies  in  the  lumen  as  a  thick  fluid  or  semi-fluid  mass.  It  is 
seen  to  have  a  peripheral  layer  of  lighter  material  which  may  possibly 
be  the  fluid  that  is  used  to  attach  the  main  thread  by,  or  may  be  a  stage 
in  the  elaboration  of  the  silk.  The  secreted  material  is  used  by  forcing 
out  a  compressed  end  of  this  mass  and  passing  it  through  the  spinneret. 
It  is  used  not  only  to  build  a  protecting  cocoon,  but  also  to  assist  the 
animal  to  adhere  and  progress  (in  many  ways). 

Technic.  —  All  these  tissues  cut  fairly  well  with  ordinary  fixation 
and  paraffin  sectioning.  It  is  always  important  to  keep  the  secretion 
in  situ,  and  to  embed  and  cut  so  carefully  that  it  does  not  fall  out  of  the 
sections  or  shrink  so  as  to  distort  the  pictures  produced.  The  combined 
paraffin  and  celloidin  process  is  of  use  when  the  material  is  fragile;  and 
when  it  is  tough  and  refractory  the  celloidin  process  alone  is  of  most  use. 
In  staining,  it  is  well  to  use  some  stain  that  will  differentiate  the  secretion 
material  strongly.  Most  stains  do  this  more  or  less,  but  some  of  the 
diffuse  analine  dyes,  as  cosine,  are  particularly  favorable.  With  such 
stains  the  finer  strands  of  the  material  can  be  traced  to  the  cells  which 
produced  them. 

LITERATURE 

SCHNEIDER,  K.  C.     "Lehrbuch  der  Histologie,"  S.  282,  1902. 

BOURNE,  G.  C.     "On  the  Structure  of  ^Enigma  cenigmatica,"  Quart.  Journ.  Mic.  Sc.  N.  S., 

No.  202  (Vol.  LI,  Part  II),  1907.     See  p.  282  on  byssus  gland. 
GILSON,  G.     "Recherches  sur  les  cellules  secretantes,"  "La  soie  et  les  appareils  serici- 

genes,  I,  Lepidoptera,"  La  cellule,   1890,   p.   115.     "II,  Trichopteres,"  La  cellule, 

1893,  p.  71. 


CHAPTER    XXI 
TISSUES   OF    REPRODUCTION,    GENERAL    OUTLINE 

AMONG  multicellular  animals  and  plants  the  individual  exists  for  a 
longer  or  a  shorter  period  and  then  perishes.  Certain  cells,  however, 
of  these  multicellular  organisms  may  separate  from  the  parent  individual 
and,  under  proper  conditions,  give  rise  to  new  individuals.  Through 
these  cells,  then,  an  unbroken  chain  of  living  cells  is  perpetuated.  Cells 
so  functioning  are  known  as  the  reproductive  cells. 

Reproductive  cells  are  of  two  kinds,  asexual  and  sexual.  The  asexual 
reproductive  cells  are  known  as  spores.  It  is  characteristic  of  them 
that  they  may  develop  directly,  without  conjugation,  into  a  new  indi- 
vidual. Asexual  reproductive  cells  and  their  organs  are  found  in  plants 
and  certain  very  low  forms  of  animals.  We  shall  consider  them  only 
in  the  pollen  cells  of  Magnolia. 

The  sexual  reproductive  cells  are  known  as  gametes.  It  is  charac- 
teristic of  gametes  that  without  the  union  of  two  gametes  or  their  equiva- 
lent, a  new  individual  will  not  be  developed.  Certain  apparent  excep- 
tions to  this  idea  can  be  reconciled  with  it. 

These  gametes  are  different  from  any  other  cell  or  group  of  cells  that 
may  separate,  bud,  or  divide  from  the  body  to  form  a  new  individual, 
in  that  they  go  through  a  peculiar  process  called  maturation,  which 
involves  two  cell  divisions  and  a  reduction  of  the  number  of  the  chro- 
mosomes by  one  half.  In  this  case  each  cell  does  not  develop  into  a 
new  individual  by  itself,  but  joins  with  another  reproductive  cell,  de- 
rived usually  from  another  individual  of  the  same  species,  and  by  a 
process  analogous  to  a  reversal  of  the  process  of  amitotic  cell  division 
the  two  unite  and  become  one  cell,  with  the  full  number  of  chromosomes, 
which  represents  the  beginning  of  a  new  individual  of  the  species.  This 
union  is  known  as  conjugation. 

Every  species  of  animal  has  its  own  kind  of  reproductive  cell,  but  there 
also  are  two  forms  of  this  cell,  a  male  and  &  female  form.  It  is  always  a 
male  and  a  female  cell,  usually  from  the  same  species,  that  thus  unite  to 
form  the  new  individual,  and  besides  any  deeper  or  more  significant 
difference  between  the  two  there  is  nearly  always  the  comparatively 
superficial  difference  that  the  female  cell,  called  the  ovum,  is  the  larger 

418 


REPRODUCTION  419 

and  provides  a  store  of  food  material  for  the  first  part  of  the  future 
organism's  life,  while  the  male  cell  or  spermatozoon  is  the  smaller  and 
has  organs  of  motion  which  provide  the  means  of  moving  through  the 
usually  short  distance  that  separates  the  two  when  they  are  deposited 
for  union,  the  female  cell  being  passive. 

When  united,  the  ovum  and  the  spermatozoon  form  a  single  cell,  the 
oosperm  or  zygote,  of  at  least  equal  value  quantitatively  and  qualitatively 
to  any  cell  in  either  of  the  parent  organisms.  This  oosperm  must  be 
looked  upon  as  the  ideal  cell,  representative  of  its  species,  and  it  is  by 
its  subsequent  divisions  and  the  differentiation  of  most  of  its  descendants, 
the  somatic  cells,  that  the  new  organism  is  formed. 

Some  of  its  descendants,  however,  do  not  differentiate  but  divide  each 
time  into  cells  exactly  homologous  to  the  oosperm  (Fig.  379).  These 
are  the  reproductive  cells  or  germ  cells  of  the  organism  and  they  form, 
as  it  can  now  be  seen,  an  unbroken  series  from  one  generation  to  the 
next.  They  become  smaller  by  dividing,  but  they  divide  nearly  all  their 
parts  equally  and  are  always  the  same.  They  become  ready  in  the 
adult  organism  to  grow  in  size,  to  mature,  and  to  again  part  with  their 
parent  body,  which  they  leave  to  die  while  they  unite  with  another  repro- 
ductive cell  to  form  a  new  organism.  Until  the  growth  and  maturation 
period  arrives,  however,  the  cells  can  be  distinguished  neither  as  male 
nor  female,  although  their  sex  is  probably  already  determined.  They 
lie  in  more  or  less  compact  masses  in  various  parts  of  the  bodies  of 
different  animals,  and  form,  together  with  connective  tissue  and  other 
tissues,  organs  called  the  gonads. 

The  gonad  may  be  a  collection  of  tissues  which  develop  de  novo  each 
breeding  season  to  hold  the  reproductive  cells  or  it  may  be  a  very  per- 
manent organ.  It  usually  has  associated  with  it  a  great  complexity  of 
accessory  tissues  and  organs  (genitalia)  intended  to  facilitate  the  union 
of  the  spermatozoa  with  the  ova  and  to  further  aid  them  in  their  repro- 
ductive career  (brood  pouches).  A  gonad  containing  male  cells  is 
known  as  a  testis  and  one  containing  female  cells  as  an  ovary.  Both 
male  and  female  reproductive  cells  may  occur  in  the  same  gonad  or  in 
different  gonads  in  the  same  individual,  in  which  case  the  species  is 
known  as  a  monoecious  one.  When  the  male  and  female  cells  are  found 
in  separate  individuals  it  is  known  as  a  dioecious  species.  Both  cases  are 
common.  The  higher  and  more  specialized  kinds  of  animals  are  usually 
dioecious.  The  moncecious  forms  seldom  unite  an  ovum  and  a  sperma- 
tozoon from  a  single  individual  to  form  an  offspring.  An  exchange  of 
sperm  between  two  individuals  is  the  usual  method. 

The  reproductive  cells  are  the  important  or  specific  cells  of  a  gonad, 
while  there  are  several  other  kinds  of  cells  that  play  secondary  but 
necessary  parts  in  its  structure.  Certain  cells  that  feed  the  young 


420 


HISTOLOGY 


reproductive  elements  are  called  the  nurse  cells  and  they  are  usually 
former  reproductive  cells  that  at  one  stage  or  another  of  their  develop- 
ment have  given  up  their  career  as  reproductive  cells  and  devoted  their 
energies  to  the  nourishment  of  their  more  fortunate  neighbors.  The 
other  gonad  structures  are  the  same  tissues  that  are  found  in  most 
organs,  as  connective,  blood,  muscle,  and  nerve  tissues.  They  play  the 
same  part  here  as  in  the  other  organs. 

Origin  of  the  reproductive  cells.  —  As  stated  before,  the  reproduc- 
tive cells  are  the  original  and  unaltered  descendants  of  the  dividing 
oosperm,  the  only  ones  of  these  descendants  that  have  not  been  differ- 


FiG.  379. — A,  second  cleavage  division  of  the  oosperm  of  Ascaris,  showing  the  first  differen- 
tiation by  loss  of  chromatin  (ch.)  in  the  somatic  cell.  B,  resulting  four  cells,  showing  the 
lost  chromatin,  ch.,  and  the  smaller  resulting  nuclei  in  the  daughter  somatic  cells.  (From 
WILSON  after  BOVERI.) 

entiated.  This  does  not  mean  that  changes  of  size  and  of  arrangement 
of  the  cell -organs  have  not  taken  place,  but  it  does  mean  that  the  cell 
has  retained  all  its  original  powers  and  the  necessary  structures  to  exert 
these  powers,  which  is  not  true  of  the  somatic  cells. 

This  loss  of  power  by  the  somatic  cell  can  even  be  demonstrated,  in 
one  case  at  least,  to  be  a  loss  of  some  part  of  its  chromatin  and  to  occur 
in  some  species  in  the  first  cell  division  or  shortly  after.  Such  a  solitary 
case  of  actual  proof  exists  in  the  nematode  worm,  Ascaris,  in  which  Boveri 
has  clearly  shown  that,  at  the  first  division  of  the  oosperm,  one  of  the 
resulting  cells  is  slightly  differentiated  from  its  sister  cell  by  a  potential 
loss  of  chromatin  that  occurs  in  the  next  division,  while  the  other  cell 
retains  every  feature  of  its  parent,  the  oosperm  (Fig.  379). 


REPRODUCTION 


421 


In  this  figure  it  can  be  seen  that,  while  the  long  chromosomes  of  the 
upper  cell  in  A  divide,  as  is  usual,  by  a  complete  splitting  into  equal 
halves,  in  B  the  chromosomes  become  separated  by  transverse  breaks 
into  many  parts,  and  that  when  the  division  occurs,  only  the  smaller 
middle  parts  are  drawn  apart  to  form  the  daughter  nuclei;  the  longer 
end-portions  being  allowed  to  remain  and  become  an  inert  part  of  the 
cytoplasm.  In  other  words,  one  has  become  differentiated  probably  by 
the  loss  of  some  mechanism  of  use  only  to  a  reproductive  cell.  It  has 
retained  every  feature  but  that  one  and  possesses  everything  necessary 
to  become  any  kind  of  a  body  cell  or  somatic  cell.  It  has  lost  something 
that  can  never  be  replaced  and  it  is  destined  to  run  a  course  of  develop- 
ment and  differentiation  that  will  end  in  death  for  all  its  descendants. 
(Read  Chapter  IV  for  certain  other  developments  of  this  idea.) 


FIG.  380.  —  Bit  of  germinal  ridge  of  a  young  Acanthias  embryo  (10  c.m.).     Shows  three  large 
reproductive  cells  in  the  germinal  epithelium.     X  1230. 


For  a  number  of  subsequent  divisions  the  reproductive  cell  continues 
to  give  off  one  somatic  cell  at  each  division,  while  the  first  somatic  cell 
can  produce  nothing  but  somatic  cells.  After  this  the  reproductive  cell 
disclaims  all  responsibility,  so  to  speak,  for  the  soma  or  body,  and  de- 
votes its  time  to  self-multiplication  and  the  production  of  a  few  accessory 
cells  that  will  be  of  use  to  it  later  in  its  life. 

In  most  other  animals  this  differentiation  of  somatic  from  repro- 
ductive cells  does  not  take  place,  as  far  as  we  can  see,  at  so  early  a  period, 
although  the  latter  can  be  traced  back  to  a  fairly  early  stage  in  many 
forms.  Figure  380  shows  a  bit  of  reproductive  tissue  in  a  very  young 
embryo  of  the  dogfish,  Acanthias,  in  which  three  germ  cells  are  for  the 
first  time  visible.  In  most  other  forms  it  is  not  until  much  later  that  the 
reproductive  cells  can  be  distinguished,  sometimes  not  until  the  body  is 
well  outlined  in  structure.  In  some  low  forms  most  of  the  somatic  cells 
appear  never  to  give  up  their  power  of  reproduction  and  are  differentiated 


422  HISTOLOGY 

sexually  at  any  time  during  the  adult  life  of  the  organism.  This  is  true 
of  most  plants  whose  growing  cells  are  also  their  potential  reproductive 
cells,  which,  when  the  proper  conditions  occur,  will  mature  into  male 
and  female  gametes.  In  this  case  we  find  an  organism  which  develops 
its  gonads  anew  at  each  reproductive  period  of  its  life.  The  same  facts 
are  true  of  many  sponges  and  coelenterates  and  other  low  animals. 

As  a  first  step  in  the  differentiation  and  higher  organization  of  such 
plants  and  low  animals  we  find  that  one  or  more  of  the  somatic  tissues 
lose  the  reproductive  power.  For  instance,  all  of  the  tissues  of  a  be- 
gonia plant,  as  stem  and  roots  and  even  leaves,  may  be  propagated  to 
form  a  new  plant  which  will  produce  reproductive  cells.  This  power  is 
lost  to  the  leaves  in  a  hickory  tree,  while  the  roots  and  stem  retain  it;  in 
a  hyacinth  and  some  lilies  and  iris,  the  roots  and  leaves  have  both  lost  it 
and  the  power  is  confined  to  certain  parts  of  the  root-stock  and  the 
flower.  It  may  even  be  lost  to  the  flower  in  some  highly  cultivated 
plants  that  bear  no  fertile  seeds. 

The  preceding  paragraphs  will  themselves  serve  to  harmonize  to  a 
large  degree  the  view  they  express  with  a  second  view  regarding  the 
origin  of  the  reproductive  cells :  that  they  are  not  cells  of  a  separate  line 
of  descent,  retaining  powers  that  the  somatic  cells  have  lost,  but  that 
they  may  arise  from  the  body  tissues  (usually  described  as  mesodermal 
tissues)  by  processes  of  differentiation  similar  to  those  of  any  of  the 
other  tissue  cells  in  the  various  tissues.  These  supposedly  antagonistic 
cases  merely  show  a  later  somatic  differentiation:  that  the  final  differ- 
entiation may  never  occur,  or  it  may  be  early,  or  it  may  be  deferred  until 
a  comparatively  late  period. 

As  to  the  time  at  which  sex  is  determined  there  appears  to  be  more 
doubt  and  wider  divergence  of  views  as  time  passes  and  investigation 
along  new  and  old  lines  proceeds.  The  idea  that  a  high  degree  of  nour- 
ishment during  early  embryonic  life  resulted  in  a  majority  of  females 
and  a  low  degree  in  more  males  has  been  shown  to  mostly  mean  a  prior 
killing  off  of  young  females  by  starvation.  Statistical  methods  and 
experiment  along  other  lines  have  failed  to  throw  light  on  the  matter. 
Our  chief  hope  of  definite  knowledge  of  more  immediate  causes  seems 
to  lie  at  present  in  certain  cytological  investigations  on  insects.  This 
will  be  brought  out  in  the  following  parts. 

LITERATURE 
WILSON,  E.  B.     "The  Cell  in  Development  and  Inheritance."     New  York,  1900. 


MALE  REPRODUCTIVE    CELLS  423 


GROWTH   AND    DEVELOPMENT    OF   THE   MALE    REPRODUCTIVE 

CELLS 

The  first  stages  of  a  male  reproductive  cell  are  represented,  as  are 
those  of  the  female,  by  a  rather  larger  and  clearer  mesodermal  or  ecto- 
dermal  cell,  which  stands  out  from  its  fellows  and  must  be  identified 
by  its  position  and  the  surrounding  tissues  rather  than  by  its  structure. 
Unlike  the  early  ovum  this  cell  has  no  great  amount  of  food  to  store 
up  in  its  body  during  the  first  part  of  its  development.  Certain  of  its 
fellows,  however,  are  differentiated  at  an  early  period  to  act  as  nutri- 
tive cells  to  it  during  the  last  part  of  its  development.  As  in  the  female, 
they  are  termed  the  nurse  cells  or,  sometimes,  the  Sertoli  cells. 

The  male  reproductive  cells  are,  at  first,  scattered  through  the  future 
gonad  or  on  its  surface.  As  development  advances  they  segregate  into 
groups  which  are  either  rounded  masses  or  elongate  rod-like  regions,  as 
for  instance  the  seminiferous  tubules  of  mammals.  We  shall  call  these 
groups,  irrespective  of  their  shape,  the  spermatic  lobules.  As  the  sperm 
cells  ripen  the  lobule  either  acquires  a  duct  which  conducts  the  semen 
away  or  else  it  ruptures  and  discharges  its  ripe  contents  into  a  body 
cavity  or  out  into  the  surrounding  water. 

Most  of  these  lobules  are  solid  masses  while  the  reproductive  cells  are 
young,  and  some  of  them  continue  so  until  the  sperm  is  ripe  and  ready 
to  be  discharged,  when  the  entire  mass  is  allowed  to  flow  into  the  sperm 
channels  by  the  rupture  of  the  lobular  wall.  Other  lobules,  which  are 
solid  at  first,  later  acquire  a  lumen.  It  is  only  when  the  spermatozoa 
are  beginning  to  mature  that  the  lumen  appears  in  the  center.  The 
presence  of  this  lumen  leaves  the  reproductive  cells  which  line  the  lobule 
lying  in  a  single  or,  more  often,  multiple  row  on  the  capsule,  and  we 
shall  hereafter  refer  to  them  in  this  condition  as  the  reproductive  epithe- 
lium. This  reproductive  epithelium  is  further  divided,  in  practically 
all  testes  of  well-differentiated  animals,  into  a  series  of  cell  groups  which 
are  of  greater  significance  and  more  fundamental  in  character  than  the 
lobule.  The  lobule  is  more  properly  an  anatomical  feature,  sometimes 
small,  as  in  most  Crustacea,  and  largest  perhaps  in  some  of  the  mammals, 
where  it  forms  the  long  seminiferous  tubules  mentioned  above.  These 
more  fundamental  groups  of  male  reproductive  cells,  which  we  shall 
call  the  sperm  columns,  are  smaller  groups  based  upon  some  nutritive 
relations  to  the  nurse  cells.  They  are  also  determined  probably  by  the 
time  that  some  particular  group  of  spermatogonia  initiates  the  matura- 
tion process.  The  sperm  column  is  usually  associated  with  a  single 
nurse  cell  or  Sertoli  cell,  although  it  may  rarely  have  more  than  one  such 


424  HISTOLOGY 

nourishing  cell.  The  lumen  of  a  lobule  will  be  considered  as  distal  in 
direction  and  the  capsule  as  proximal. 

The  male  gonad  differs  histologically  according  to  the  season  and 
the  ways  in  which  the  sperm  is  matured.  Some  organisms  mature  but 
one  lot  of  sperm  in  a  lifetime,  and  others  mature  it  from  a  very  different 
and  newly  developed  testis  each  year.  In  such  a  form  we  are  apt  to 
find  a  lack  of  the  sperm  column  segregation  and  to  find  that  each  lobule 
matures  all  its  sperm  at  once  and  in  a  mass.  Such  a  lobule  is  not  a  per- 
manent structure,  but  is  destroyed  immediately  after  the  discharge  of 
the  spermatozoa. 

In  the  other  animals  the  sperm  may  be  produced  for  long  periods  or 
even  continuously,  as  in  man.  Here  the  lobule  is  usually  a  permanent 
structure  and  a  residuum  of  living  reproductive  cells,  as  spermatogonia, 
is  always  to  be  found  on  the  basement  membrane.  At  certain  periods, 
determined  in  man  by  "waves"  of  successive  maturation  periods  which 
pass  down  the  tubules,  some  of  these  spermatogonia  begin  to  undergo 
maturation.  As  they  begin  to  mature  and  develop  they  leave  their 
basal  position  and  move  distally  in  successive  layers,  meanwhile  going 
through  the  maturation  stages,  until  when  they  arrive  at  the  lumen 
they  become  functional  male  reproductive  cells. 

The  nurse  cells  commonly  remain  on  the  basement  membrane. 
This  obliges  the  growing  spermatids  to  move  to  them  and  remain  in  a 
proximal  position  until  discharged.  All  the  cells  of  a  single  sperm  col- 
umn commonly  mature  together.  In  the  skate  we  find  a  long,  sea- 
sonal, sperm-production  period  during  which  a  series  of  new  spermatic 
lobules  are  being  continually  formed  from  a  germinative  center.  As 
these  mature  they  move  away  from  this  center  until,  when  ripe,  they 
are  ruptured  and  destroyed  at  the  surface  of  the  testis,  setting  the  sper- 
matozoa free  into  the  seminal  ducts.  These  lobules  show  a  well- 
defined  sperm  column  arrangement. 

The  development  of  a  spermatogonium  into  four  ripe  spermatozoa  is 
one  of  the  most  interesting  of  known  cytological  processes.  The  stages, 
divisions,  etc.,  through  which  they  go  are  exactly  homologous  to  those  to 
be  later  described  for  the  female  reproductive  cells.  We  shall  describe 
these  stages  simply  and  shortly  at  first. 

Beginning  on  the  basal  layer  as  a  spermatogonium,  the  cells  await  the 
breeding  season,  and  having  gone  through  the  contraction  stage  or  synizesis 
each  one  grows  in  size  to  become  a  spermatocyte  of  the  first  order.  This 
spermatocyte  now  lies,  as  a  rule,  in  the  second  layer  of  the  reproductive 
epithelium  and  rapidly  goes  through  with  its  first  reduction  division, 
which  results  in  the  production  of  two  spermatocytes  of  the  second  order. 
The  chromatin  is  arranged  and  divided  as  will  be  described  in  detail 
below  and,  sometimes  without  re-forming  their  nuclei,  the  second  sper- 


MALE  REPRODUCTIVE    CELLS  42$ 

matocytes  divide  again,  producing  four  spermalids  which  each  possess 
one  half  the  number  of  chromosomes  that  the  spermatogonium  did. 
These  four  spermatids  all  develop  without  further  divisions  into  sper- 
matozoa. We  should  now  examine  more  closely  as  to  what  happens 
during  the  reduction  divisions. 

Maturation  is  a  phenomenon  common  to  both  spermatogenesis  and 
oogenesis  and  is  an  essentially  similar  process  in  either  event.  Two 
rapidly  succeeding  divisions,  the  reduction  divisions,  constitute  the 
important  phase  of  maturation.  These  divisions  effect  a  reduction 
of  the  number  of  chromosomes  by  one  half,  and  involve  primarily  a 
quantitative  equal,  frequently  combined  with  a  qualitatively  dissimilar 
distribution  of  the  fission  products  (chromosomes)  among  the  resulting 
cells.  The  actual  numerical  reduction  of  the  chromosomes  has  already 
occurred  during  synapsis  when  the  chromosomes  united  into  pairs, 
forming  bivalent  chromosomes,  or  several  may  even  have  combined  to 
form  plurivalent  chromosomes. 

It  is  now  believed  that  the  pairs  of  chromosomes  in  synapsis  are  com- 
posed of  maternal  and  paternal  elements  and  that  their  union  repre- 
sents the  final  stage  in  the  fertilization  process  which  resulted  in  the  origin 
and  development  of  the  organism  whose  germ  cells  are  now  in  synapsis. 
It  is  clearly  known  in  several  cases  that  the  maternal  and  paternal  chro- 
mosomes do  not  fuse  at  fertilization  nor  during  the  several  succeeding 
segmentation  divisions,  and  it  is  very  probable  that,  in  the  germ  cells 
at  least,  the  chromosomes  from  the  two  parents  do  not  fuse  until  synapsis. 

Synapsis,  as  described  in  many  insects  and  plants,  usually  takes 
place  during  the  telophase  of  the  final  oogonial  or  spermatogonial  divi- 
sion, though  it  has  been  observed  to  occur  slightly  earlier  or  later,  some- 
times even  during  the  synizesis  (contraction  phase)  of  the  chro matin 
at  the  beginning  of  the  growth-period  of  the  oocyte  or  spermatocyte. 
According  to  the  observations  of  various  investigators  in  the  various 
animal  and  plant  groups,  synapsis  may  be  an  end  to  end  union  of  the  ele- 
ments of  a  pair  of  chromosomes  or  they  may  unite  side  by  side.  If  they 
unite  in  the  former  way,  we  have  a  case  of  telosynapsis;  if  by  the  latter 
method,  a  case  of  parasynapsis. 

It  is  known  on  good  evidence,  in  some  cases,  that  one  of  the  matura- 
tion divisions  separates  entire  chromosomes,  and  along  the  plane  of 
their  previous  union  in  synapsis.  It  is  only  reasonable  to  suppose  that 
the  process  is  similar  also  in  the  more  obscure  cases.  It  is  therefore 
essential  for  a  correct  interpretation  of  the  maturation  phenomena  and 
the  reduction  divisions  that  we  know  how  the  chromosomes  united  in 
synapsis.  This  is  known  in  but  few  cases.  A  maturation  division  that 
separates  bivalent  chromosomes  into  qualitatively  dissimilar  halves  is 
known  as  a  reducing  division;  a  division  that  separates  chromosomes 


426  HISTOLOGY 

into  qualitatively  and  quantitatively  equal  halves  is  called  an  equation 
division.  If  the  division  is  of  the  ordinary  mitotic  type,  it  is  known  as 
homeotypic;  if  the  prophase  of  the  division  is  characterized  by  various 
ring-  and  cross-shaped  chromosomes,  the  mitosis  is  said  to  be  heterotypic. 
If  the  reducing  division  (frequently  heterotypic)  precedes  the  equation 
division,  the  case  is  known  as  prereduction;  the  reverse  condition  presents 
a  case  of  postr  eduction.  Both  methods  have  been  observed  among  ani- 
mals and  plants  and  with  about  equal  frequency.  Indeed,  in  some 
cases  investigators  disagree  as  to  which  is  the  method  in  the  same  species 
of  animal.  In  very  many  recorded  and  well-authenticated  cases,  how- 
ever, one  of  the  maturation  divisions  is  a  reducing  division  and  the  other 
is  an  equation  division. 

In  spermatogenesis,  where  the  four  cells  resulting  from  the  matura- 
tion divisions  of  a  spermatogonium  are  all  functional,  the  reducing 
division  effects  a  qualitative  inequality  between  the  two  pairs  of  fission 
products,  as  demonstrated  by  Wilson  in  certain  Hemiptera;  this  dis- 
similarity may  be  in  part  a  sex-determining  factor,  as  McClung  first 
suggested.  The  reducing  division  in  oogenesis,  where  only  one  of  the 
four  cells  resulting  from  the  maturation  process  remains  functional, 
effects  the  loss  of  chromatin  to  the  ovum  that  may  have  represented  sex 
determinants  and  various  other  ancestral  hereditary  characters.  Ac- 
cording to  Weismann  and  his  followers,  the  gist  of  the  maturation  phe- 
nomenon lies  in  the  redistribution  of  the  morphological  representatives 
of  hereditary  characters,  and  offers  the  basis  for  variation  and  selection. 

To  take  up  a  concrete  example,  let  us  consider  the  maturation  proc- 
esses of  the  starfish,  Aster ias  forbesii,  where  the  chromosomes  are  all 
characteristically  dumb-bell-shaped  throughout  the  maturation  divi- 
sions. Here  the  somatic  number  of  chromosomes  is  about  36.  In  syn- 
apsis,  which  probably  occurs  during  synizesis,  this  number  is  reduced 
to  18,  so  that,  with  possibly  one  or  two  exceptions,  the  resulting  com- 
pound chromosomes  are  bivalent.  These  chromosomes  are  typically 
bi-lobed.  Since  the  details  of  synapsis  in  this  particular  case  have  not 
yet  been  observed,  we  have  no  clew  as  to  what  the  maturation  divisions 
mean  where  two  longitudinal  fissions  are  known  to  occur.  But  let  us 
consider  the  various  possibilities. 

If  the  chromosomes  united  end  to  end  in  synapsis  (telosynapsis) 
and  condensed  into  a  bi-lobed  chromosome,  so  that  one  lobe  is  A  (the 
parental  chromosome)  and  the  other  is  B  (the  maternal  chromosome), 
then  the  first  longitudinal  division  yields  chromosomes  that  must  be 
represented  by  AB,  and  the  division  was  an  equation  division.  The 
second  longitudinal  divisions  would  again  result  in  chromosomes  AB, 
and  no  true  reduction  would  have  taken  place.  Suppose  the  chromo- 
somes to  have  fused  side  by  side  in  synapsis  (parasynapsis)  and  condensed 


MALE  REPRODUCTIVE    CELLS  427 

into  a  bi-lobed  chromosome  so  that  both  lobes  must  be  represented  by 
AB.  The  first  longitudinal  division,  if  the  separation  really  takes  place 
along  the  line  of  original  fusion,  yields  two  chromosomes,  A  and  B,  and 
there  has  been  a  true  reduction,  and  the  case  is  one  of  pre-reduction. 
The  second  longitudinal  division  simply  divides  chromosomes  A  and  B 
into  similar  chromosomes  of  half  the  original  size,  and  the  division  is 
an  equation  division.  In  many  cases  one  of  the  maturation  divisions  is 
transverse,  but,  however  the  divisions  take  place,  the  important  fact  as 
to  whether  the  divisions  are  qualitative  or  quantitative  merely  depends 
upon  the  manner  of  union  of  the  chromosomes  in  synapsis.  These  two 
important  divisions  have  resulted  in  four  spermatids. 

The  great  temporary  differentiation,  which  the  male  reproductive 
cell  undergoes  to  adapt  itself  for  motion,  is  now  developed,  and  trans- 
forms it  from  a  spermatid  into  the  final  form,  the  spermatozoon  ( Fig.  381). 

The  spermatozoon  is  a  cell  which  has  a  variety  of  forms  in  different 
animals  and  these  variations  of  structure  can  be  best  studied  and  under- 
stood when  it  is  remembered  that  they  have  for  a  common  object  the 
transportation  of  the  important  nucleus  of  this  cell  to  the  ovum,  its 
entrance  into  this  ovum,  and  the  final  apposition  of  its  chromosomes 
with  those  of  the  ovum. 

There  are  two  ways  in  which  this  transportation  is  accomplished : 
by  the  amoeboid  movements  of  an  undifferentiated  cytoplasm  as  in  the 
spermatozoa  of  some  Crustacea  (Fig.  381,  H);  and  by  the  development 
of  one  or  more  flagella  or  permanent  cytoplasmic  processes  which  pro- 
pel the  cell  by  swimming  movements  instead  of  by  crawling,  as  must  be 
the  case  in  amoeboid  cells  (Fig.  381,  all  forms  except  H).  By  far  the 
larger  number  of  spermatozoa  are  of  the  swimming  kind,  and  most  of 
these  are  propelled  by  a  single  strong  flagellum.  In  the  analogous  male 
reproductive  cells  of  plants  there  are  oftener  two  flagella.  We  shall 
first  concern  ourselves  with  the  structure  of  the  more  typical  flagellate 
spermatozoon,  as  shown  in  the  diagram  to  the  right  in  Figure  381. 

In  this  form  the  nucleus  appears  as  a  compressed  oval  body  which 
is  placed  in  an  anterior  cytoplasmic  enlargement,  the  head.  The  nucleus 
is  composed  of  the  reduced  number  of  individual  chromosomes,  of 
course,  but  they  are  indistinguishable  at  this  time,  forming  a  solid  chro- 
matic content  of  the  nucleus.  The  head  and  its  contained  nucleus  is 
not  always  oval,  but  may  be  much  elongate,  corkscrew-shaped  (381,  D), 
or  formed  like  a  horseshoe  (381,  C).  The  head  terminates  in  a  cap- 
like  structure  frequently  sharp,  occasionally  blunt,  called  the  acrosome. 

The  heaviest  mass  of  the  cytoplasm  lies  behind  the  head,  and  is  known 
as  the  middle-piece.  It  is  the  cell  body  of  the  spermatozoon  and  contains 
the  future  centrosome,  if  this  body  is  carried  over  structurally  from 
generation  to  generation.  The  middle-piece  is  developed  from  the 


428 


HISTOLOGY 


nebenkern  or  accessory  nucleus,  a  body  found  in  the  spermatid.  The 
middle-piece  contains  a  round  body  which  is  sometimes  double  and  is 
called  the  end-knob.  It  is  chromatic,  and  lies  just  behind  the  nucleus. 


Apical  body  or  acrosome. 

Nucleus. 

End-knob. 

Middle-piece. 

Envelope  of  the  tail. 
Axial  filament. 


End-piece. 


FIG.  381. — Labeled  diagram  of  a  typical  flagellate  spermatozoon,  and  figures  of  eleven  actual 
spermatozoa  to  typify  some  of  the  principal  forms.  A,  the  badger,  Meles;  B,  a  bat,  Vesper- 
ugo;  C,  opossum,  Didelphys ;  D,  a  bird,  Muscicaps ;  E,  a  sturgeon,  Acipenser ;  F,  a  crab, 
Porcdlana;  G,  the  lobster,  Homarus ;  H,  a  crustacean,  Polyphemus  /  /,  an  insect,  Calathus ; 
J,  a  salamander,  Triton;  K,  a  snake,  Coluber.  (From  WILSON  after  WILSON  (K  and  C); 
BALLOWITZ  (A,  B,  D,  E,  7,  and  J);  ZACHARIAS  (H);  GROBBEN  (F);  and  HERRICK  (G).) 


There  is  often  a  ring-shaped  structure  associated  with  it.    The  middle- 
piece  is  sometimes  much  larger  than  the  head  (Fig.  381,  B). 

Running  distally  from  the  end-knob,  in  our  typical  spermatozoon,  is 
a  thread-like  structure  known  as  the  axial  filament.  This  filament 
becomes  continuous,  at  the  end  of  the  middle-piece,  with  the  central 
portion  of  a  single,  long  flagellum. 


MALE  REPRODUCTIVE  CELLS  429 

This  flagellum,  or  tail,  as  it  is  known,  consists  of  a  strong  axial  fila- 
ment which  shows  a  fibrillar  structure  like  that  of  smooth  muscle.  The 
cytoplasm  covers  it  as  a  sheath,  except  for  a  part  of  its  distal  end,  which 
has  been  called  the  end-piece. 

Developed  in  or  by  the  cytoplasmic  sheath  is  a  particular  struc- 
ture intended  to  give  the  tail  a  proper  resistance  to  the  fluid  through 
which  it  swims.  This  is  the  fin,  and  may  assume  a  number  of  peculiar 
forms.  It  is  usually  a  filament  or  ribbon  of  considerable  length,  pro- 
jecting as  a  flange  from  the  cytoplasmic  sheath  around  which  it  is  spi- 
rally wound. 

Many  spermatozoa  are  of  a  widely  different  type  of  structure.  The 
simplest  is  an  amoeboid  form  found  in  some  Crustacea.  Other  Crustacea 
have  very  peculiar  kinds,  all  of  which  seem  to  be  constructed  on  a  radial 
plan,  with  from  three  to  twelve  or  more  processes  that  cannot  be  called 
cilia  or  flagella  on  account  of  their  structure.  They  are  permanent 
cytoplasmic  processes. 

The  changes  through  which  the  spermatid  develops  into  the  sperma- 
tozoon are  well  demonstrated  in  the  following  pages.  We  shall  outline 
them  in  a  few  words  at  this  point. 

The  spermatid  is,  at  first,  a  very  ordinary-looking  cell,  rather  smaller 
in  size  than  the  average  and  with  no  suggestion  about  it  of  the  sperma- 
tozoon form.  Its  first  step  in  development  is  the  appearance  on  one  edge 
of  a  dot,  the  future  end-knob,  from  which  a  tiny  filament  grows  out  dis- 
tally.  As  the  filament,  which  is  the  future  tail,  and  the  granule  giow  in 
size,  they  push  in  toward  the  nucleus,  which  the  end-knob  almost  touches. 
A  ring  forms  around  the  proximal  part  of  the  tail  which  now  becomes 
the  middle-piece,  and  the  nucleus  becomes  compact  and  apparently  loses 
its  reticular  chromatin.  It  moves  into  an  eccentric  position  in  the  cyto- 
plasm, and  this  region  later  becomes  the  head  of  the  spermatozoon. 
The  nucleus  may  assume  a  number  of  forms  before  finally  becoming 
the  head  of  the  adult  spermatozoon.  The  ring  usually  elongates  and 
becomes  spirally  arranged  about  the  tail.  The  axial  filament  has 
been  shown  to  originate  from  a  centrosome  ray.  Sometimes  a  sec- 
ondary spermatocyte  makes  a  weak  attempt  to  form  a  tail  in  this 
way. 

During  this  elaborate  development  of  highly  differentiated  motor 
structures,  the  cell  shows  a  varying  ability  in  the  power  to  nourish  itself. 
Many  spermatids  apparently  find  no  difficulty  in  securing  food  from  the 
surrounding  fluids,  while  others  resort  to  the  same  methods  that  most 
developing  ova  do,  and  attach  themselves  to  a  nurse  cell.  In  this  case, 
however,  one  nurse  cell  becomes  the  feeder  of  many  spermatids  which, 
on  account  of  their  small  size,  need  but  little  food.  The  connection  is 
usually  not  established  until  the  spermatid  stage  is  somewhat  advanced. 


430 


HISTOLOGY 


The  relation  is  diffuse  in  some  few  animals  (and  most  plants).  This 
diffuse  connection  by  proximity  can  be  well  seen  in  the  pollen  sac  of 
Magnolia,  where  the  outer  members  of  a  homogeneous  cord  of  cells  be- 
come the  nurse  cells  and  feed  the  inner  cells  of  the  same  mass,  which 
become  the  pollen  cells  (see  Fig.  383).  These  pollen  cells  are  formed 
in  essentially  the  same  way  as  are  animal  spermatozoa. 

The  diffuse  relation  between  the  source  of  nourishment  and  the  nour- 
ished spermatids,  somewhat  as  indicated  in  the  above  example,  is  the 
most  general  condition  among  lower,  and  especially  the  smaller,  inverte- 
brate animals. 

Good  concrete  examples  of  a  more  specialized  connection  are 
to  be  seen  in  the  vertebrate  animals.  Here  the  spermatids,  soon 
after  their  development  into  spermatozoa  is  begun,  form  an  attach- 
ment with  a  cell  which  lies  near  the  basal  layer  of  the  reproductive 
epithelium. 

They  are  drawn  or  move  down  between  the  other  cells,  and  the  head, 
of  each  one  of  a  large  group,  becomes  partly  embedded  in  and  firmly 
attached  to  the  cytoplasm  of  this  nurse  cell  which,  in  the  mammals,  is 
called  a  Sertoli  cell.  Here  they  remain  until  maturity,  when  they  are 

released  and  set  free  to  pass  out 
of  the  reproductive  lobule  into  the 
organs  used  for  then-  distribution. 
This  will  be  described  in  con- 
nection with  the  spermatogenesis 
of  the  skate. 

In  the  salamander,  Desmogna- 
thus  fusca,  a  somewhat  different 
method  is  used.  As  in  the  mam- 
mal, the  spermatids  attach  them- 
selves to  the  nurse  cell,  but  the 
very  large  nurse  cell  is  set  free 
toward  the  end  of  the  sperm  de- 
velopment and  floats  about  in 
the  lumen  of  the  lobule,  feeding 
the  spermatids  until  they  are  ripe. 
It  then  degenerates  and  is  lost, 
freeing  the  spermatozoa.  Figure 
382  shows  this  condition. 

The  first  series  of  developing 
reproductive   cells  that  we   shall 
study  will  be  in  a  plant,  Magnolia 
soulangeana.     These  asexual  re- 
productive  cells  only  indirectly  give  rise  to  male  reproductive  cells. 


FIG.  382.  —  Two  sperm  nurse  cells  from  the  sala- 
mander Desmognathus  fusca .  Each  n  urse  cell 
has  a  group  of  half-developed  spermatozoa  at- 
tached and  is  feeding  them  during  their  growth. 


MALE  REPRODUCTIVE    CELLS 


431 


Here  it  is  most  clearly  to  be  seen  that  four    cells   result  from  the 
two  reduction  mitoses. 

This  can  be  demonstrated  clearly  and  plainly  because  at  the  begin- 
ning of  this  period  each 
mother  cell  becomes  incased 
in  a  tough-walled  sac  or  cell- 
wall,  and  remains  in  this  same 
envelope  until  its  four  descen- 
dants become  full-grown  pol- 
len grains.  This  feature 
serves  to  show  that  the  proc- 
ess of  reduction  in  plants 
and  animals  is  much  the 
same  and  is  possibly  an  ho- 
mologous process. 

The  origin  of  the  pollen 
mother  cells  is  a  more  or  less 
late  (in  the  life  of  the  indi- 
vidual tree)  differentiation, 
which  occurs  each  year  from 
the  ever  young  cells  of  the 
plant's  upper  growing  point. 
As  member  after  member,  like 
leaf,  bract,  petal,  etc.,  is  pro- 
duced by  these  cells,  a  time 
comes  when  the  season  or 
stage  of  growth  determines 
the  differentiation  of  a  group 
of  members  called  the  flower, 
and  in  this  group  are  certain 
members  called  the  stamens. 

Four  longitudinal,  parallel 


FIG.  383.  —  A,  outer  nurse  cells  in  pollen  sac  of 
Magnolia  soulangeana  beginning  to  differentiate 
from  inner  reproductive  cells.  B,  nurse  cells  in 
their  vigor  feeding  pollen  mother  cells,  one  of  which 
is  shown  in  outline.  One  nurse  cell  in  mitosis. 
C,  nurse  cells  degenerating  after  pollen  cells  are 
formed.  Two  groups  of  pollen  cells  in  outline. 
Low  magnification. 


regions  called  the  pollen  sacs 
are  early  marked  out  in  such 
a  stamen  member,  and  the 
cells  within  them  become  the 
primitive,  pollen-forming  cells. 
These  cells  are  all  alike  at  this 
time  (early  fall  in  Princeton)  and  lie  dormant  all  winter,  perhaps  pursuing 
a  very  slow  growth  on  the  least  cold  days.  In  February,  with  the  warm- 
ing weather,  the  pollen  sacs  begin  to  grow,  and  now  it  will  be  noticed 
that  the  central  cells  are  increasing  in  size  much  faster  than  the  two  outer 
layers.  This  is  well  shown  in  Figure  383,  A,  where  the  centrally  situated 


432 


HISTOLOGY 


cells  in  a  transverse  section  of  a  pollen  sac  are  larger  and  have  propor- 
tionally far  larger  nuclei  and  nucleoli  than  the  peripheral  cells.     These 

latter  cells  become  differen- 
tiated as  the  nurse  cells, 
forming  at  a  later  stage 
(Fig.  383,  B)  a  double  row 
of  cells  whose  nuclei  divide, 
by  amitosis  or  sometimes 
by  a  peculiar  many-cen- 
tered mitosis  seen  in  the 
figure,  into  two  nuclei, 
without  a  subsequent  divi- 
sion of  the  cytoplasmic 
body.  Some  of  them  al- 

FIG.  384. -Pollen  mother  cell  of  Magnolia  at  period  of     ready   have    tw°    nUcld   in 
completed  growth  and  before  any  reduction  processes      Figure   383,   A. 

have  set  in.    x  1800.  Meanwhile  the   central 

pollen  mother  cells  have  grown  to  the  proportional  size  indicated  in 
Figure  383,  B,  where  the  outline  of  a  full-sized  pollen  mother  cell  is 
seen  in  contact  with  the  double  layer  of  nurse  cells,  now  at  their  fullest 
size  and  vigor.  The  double  nuclei  and  the  peculiar  form  of  mitosis, 
which  is  rarely  seen  among  them,  are  well  shown  in  this  figure.  The 
ultimate  fate  of  the  nurse  cells  is  shown  in  Figure  383,  C,  which  rep- 
resents the  pollen  sac  at  the  time  that  each  pollen  mother  cell  has 
divided  by  its  two  reduc- 
tion divisions  into  four 
young  pollen  cells.  During 
this  time  the  nurse  cells 
have  evidently  been  nour- 
ishing the  reproductive 
cells,  and  now  appear  as 
a  thin  layer  of  shrunken 
cells,  which  are  soon  to 
further  disintegrate  and 
finally  to  disappear. 

A  pollen  mother  cell, 
such  as  is  outlined  in  Fig- 
ure 383,  B,  is  better  shown 


FIG.  385.  —  Pollen  mother  cell  of  Magnolia  preparing 
for  reduction  divisions,  nucleolus  vacuolated.  Chro- 
mosomes beginning  to  form.  X  1800. 


by  Figure  384,  which  shows 

one  at  the  maximum  size 

of  its  growth  and  while  it 

yet  retains  its  primitive  nuclear  structure.    The  nucleolus  is  very  large 

and  very  perfect  in  outline  and  shows  no  vacuoles.    The   skein  which 


MALE  REPRODUCTIVE    CELLS 


433 


dm. 


appears  is  made  up  of  a  chromatic  and  an  achromatic  material, 
mostly  the  latter.  It  is  smooth  and  wiry,  with  darker  masses  at  the 
intersections  of  the  strands 
of  the  skein. 

Passing  by  several  inter- 
mediate stages,  Figure  385 
shows  a  more  developed 
cell  in  which  it  is  to  be 
seen  that  the  skein  is  much 
broken  up  and  its  remains 
have  been  gathered  against 
the  nuclear  membrane. 

These  remains  have  also 
either  acquired  the  power 
of  staining,  or  they  have 

had  Other  material  added  p^.  386.— Pollen  mother  cell  of  Magnolia  with  nu- 
tO  them  tO  take  the  Stain.  clear  membrane  gone,  nucleolus  very  small,  and  chro- 
'TVu'c  lat^r  ^™e  tV,~  rnr,™  mosomes  forming.  Achromatic  fibrils  are  forming. 

Ibis  latter  seems  the  more  dntj  darker  band  of  cytophsmk material,  x  1800. 
probable  when  we  notice 

that  during  this  time  the  large  black-staining  nucleolus  has  been  going 
through  a  process  of  disintegration  by  the  formation  of  vacuoles. 
While  it  appears  larger  on  account  of  the  vacuoles,  it  is  undoubtedly 
smaller  in  bulk.  This  has  not  been  demonstrated  by  any  measure- 
ments, but  is  strongly  indicated,  if  not  proved,  by  the  immediately  im- 
pending disappearance  of  the  nucleolus  by  this  same  method.  The 

chromatic  matter  is 
thus  gathered  in  irreg- 
ular granules  around 
the  periphery  and  these 
granules  increase  in 
size  as  the  nucleolus 
decreases  in  bulk.  A 
small  portion  of  achro- 
matic material  re- 
mains in  a  central 
position.  The  whole 
nucleus  enlarges  as 
this  proceeds. 

At  the  time  at 
which  the  nucleolus  is 
dissolved,  or  shortly 
before,  a  series  of  achromatic  fibrils  appear  in  loose  formation  around 
the  edge  of  the  nucleus  whose  wall  becomes  indistinct  and  disappears. 


FIG.  387.  —  Metaphase  of  first  reduction  division  in  Magnolia. 
X  1800. 


434 


HISTOLOGY 


FIG.  388.  — Anaphase  of  first  reduction  division  in 
Magnolia.     X  1800. 


At  the  same  time  an  equatorial  band  of  darker  material  appears  in  the 

cytoplasm  surrounding  the  nucleus  (Fig.  386,  d.m.}\  this  band  is  cut  at 

two  points  and  the  sections 
appear  as  two  roughly 
crescentic  lines  of  some 
width  and  tapering  to 
blunt  points.  The  chro- 
matic material  has  mostly 
left  the  nucleus  and  been 
added  to  the  chromatin 
particles  which  have  now 
become  larger,  more  uni- 
form in  size,  and  are 
evidently  the  future  chro- 
mosomes of  the  first  reduc- 
tion  divisions.  They 
appear  in  the  next  stage 
represented  (Fig.  387)  in 

their  regular  size,  shape,  and  arrangement,  and  the  fibrils  have  been 

arranged   into  the   familiar   spindle  which   here  is  gathered    distally 

into  several  points  near  together.    The  dark  cytoplasm  material  is  now 

increased  to  form  a  complete  shell  about  the  whole  figure  which  is  just 

beginning  to  divide  its  chromatin. 

Figure  388  shows  the  division  half  done,  and  again  shows  the  differen- 
tiation of  mantle  and  spindle  fibrils.    The  spindle  fibrils  seem  to  be  fewer 

in  this  stage  than  in  a 

later    one    (Fig.    389), 

where    they    are    very 

numerous,     and     have 

already  begun  to  show 

the  equatorial  plate  that 

marks  the    position  of 

the  cell's  final  division. 

The   chromosomes   are 

still    separate    and   are 

somewhat    fused.     The 

dark  zone  of  cytoplasm 

is   diffused  and   shows 

but     a     trace     of     its 

former  presence  FIG.  389.  —  Late  anaphase  of  first  reduction  division  in 

Radiating     aster-like 

rays  reach  from  the  chromosome  masses  out  into  the  cytoplasm. 
In  Figure  390  the  two  nuclei  are  re-formed  and  have  acquired  a  very 


MALE  REPRODUCTIVE    CELLS 


435 


much  smaller  nucleolus.  A  nuclear  membrane  is  present  and  the  chro- 
matin  is  to  be  seen  as  scattered  granules  on  the  newly  formed  skein  of 
achromatic  material.  The 
cytoplasm  of  the  cell  is 
constricted  in  the  plane 
of  future  division. 

Before  this  cytoplasmic 
division  has  taken  place, 
however,  the  second  reduc- 
tion division  has  started 
(Fig.  391).  The  two  fig- 
ures of  this  process  occur 
at  the  same  time  and  usu- 
ally at  right  angles  to  one 
another.  This  is  very 
convenient  as  a  control 
for  further  observations. 
The  darker  cytoplasmic 
zone  as  seen  in  the  first 


FIG.  390.  —  Telophase  of  the  first  reduction  division  in 
Magnolia.     X  1800. 


division,  and  which  was  lost  in  the  short,  intermediate,  resting  stage, 

is  now  re-formed  around  each  of  the  two  new  figures. 

The  last  figure  (Fig.  392)  shows  the  results  of  these  two  divisions, 

four  small  cells  all  lying  in  the  same  capsule  or  cell-wall  which  was  formed 

by  the  original  pollen 
mother  cell  division.  The 
reduction  divisions  have 
thus  resulted  in  four  cells. 
The  nuclei  of  these  four 
cells  but  rarely  appear  in 
any  one  given  section, 
owing  to  the  position  of 
the  spindles  by  which  they 
were  formed  being  at  right 
angles  to  each  other.  Thus 
the  section  from  which 
Figure  392  was  drawn  was 
cut  obliquely  through  the 
group,  and  two  of  the  nu- 
clei are  only  cut  near  their 
periphery  and  present  a 
smaller  section. 
The  young  cells  now  develop  a  cell-wall  of  their  own  of  very  peculiar 

pattern,  with  external  spikes  of  knobs,  and  having  broken  out  of  the 


FIG.  391.  —  Beginning  of  the  second  reduction  division  in 
Magnolia.  The  two  spindles  have  formed  at  right  angles 
to  each  other,  thus  permitting  two  views  of  the  figure  in 
one  section.  X  1800. 


436 


HISTOLOGY 


FIG.  392.  — Youngest  stage  of  the  four  pollen  cells  of 
Magnolia,  which  are  all  inclosed  in  the  single  cell- 
wall  made  by  the  pollen  mother  cell.  X  1800. 


original  cell-wall  which  still  incloses  the  four,  they  lie  massed  in  the  sac 
to  await  its  ripening  and  rupture.    The  drying  up  of  the  scant  remains 

of  the  former  nurse  cells  sets 
them  entirely  free  from  any 
connection  with  the  pollen 
sac. 

The  spermatogenesis  of  the 
skate,  Raja  ocellata,  affords 
a  very  splendid  object  for 
demonstrating  most  of  the 
details  of  sperm  formation 
during  which  reduction  takes 
place  in  the  male  gametes  of 
animals.  It  illustrates  at  the 
same  time  several  interesting 
histological  methods  of  ar- 
rangement. The  testis  is  a 
solid  mass  of  tissue  devel- 
oped from  the  genital  ridge, 
which  is  practically  the  same  as  in  Acanthias  (see  Fig.  380).  The 
large  primitive  reproductive  cells  of  this  ridge  have  multiplied  and  are 
situated,  in  the  adult  Raja,  at  a  number  of  positions  near  the  surface  of 
the  testis.  They  remain  in  these  positions  during  the  life  of  the  animal 
and  are  constantly  dividing  so  that  they  form  small  separate  groups  of 
germ  cells  which  we  shall  call  the  germinal  centers.  In  this  center  are 
a  certain  number  of  connective-tissue  cells  as  well  as  the  reproductive 
cells. 

Each  of  these  primitive  reproductive  cells  is  surrounded  by  several 
of  the  connective-tissue  cells  (Fig.  393). 
On  the  boundaries  of  the  germinal  center 
the  reproductive  cells  may  be  seen  divid- 
ing inside  their  enlarging  connective-tis- 
sue cell  coverings  as  in  Figure  394,  where 
each  of  the  two  groups  of  cells  shown 
came  from  a  single  reproductive  cell  such 
as  is  shown  in  Figures  380  and  393.  Thus 
each  reproductive  cell  comes  to  form  a 
spermatic  lobule,  which  grows  in  size  and 
matures  its  contents  as  it  is  pushed  away 
from  the  germinal  center  by  the  forma- 
tion of  new  lobules  from  this  center.  This 
whole  mass  of  many  lobules  is  surrounded  by  a  capsule  of  connective 
tissue,  forming  one  of  the  lobes  of  the  testis.  At  the  periphery  of 


FIG.  393.  —  Two  primitive  repro- 
ductive cells  (pre-spermatogonia) 
in  their  resting  stage  in  the  adult 
testis  of  Scyllium.  Each  is  sur- 
rounded by  a  few  connective-tissue 
cells.  (After  MOORE.) 


MALE  REPRODUCTIVE    CELLS 


437 


FIG.  394.  —  Two  very  young 
spermatic  lobules  in  the  tes- 
tis  of  Raja  ocellata.  The 
larger  one  shows  a  begin- 
ning of  the  lumen  (/.)  and  a 
differentiation  of  nurse  cells 
and  spermatogonia.  X  1000. 

These  latter  become  the 


these  lobes  the  sperm  is  constantly  maturing  during  the  breeding 
season  and  is  then  thrown  off  and  collected  for  use  through  the  vas 
deferens.  When  once  a  good  section  through  a 
germinal  center  is  found,  it  is  comparatively 
easy  to  follow  all  the  stages  of  sperm  develop- 
ment by  their  comparative  distances  from  this 
point  as  well  as  their  actual  structure. 

The  single  primitive  reproductive  cells  seen 
in  Figure  393  are  probably  a  peculiar  stage  in 
themselves.  We  may  call  them  the  pre-sper- 
matogonia.  Each  one  is  the  originator  of  a 
single  lobule,  and  all  the  reproductive  and 
nurse  cells  in  it.  Its  covering  of  several  con- 
nective-tissue cells  is  a  separate  structure,  and 
forms  all  subsequent  capsule  tissues.  During 
the  first  divisions  as  pictured  in  Figure  394  it 
can  be  seen  that  the  divisions  of  the  reproduc- 
tive cell  have  resulted  in  what  can  already  be 
distinguished  as  larger,  round,  nucleated  sper- 
matogonia, and  smaller  cells  with  oval  nuclei, 
nurse  cells. 

In  the  skate  and  all  elasmobranch  fishes  we  have  a  case  of  early  for- 
mation of  the  lumen  in  each  lobule.  This  was  already  indicated  in  the 
larger  lobule  of  Figure  394,  and  it  is  now  very  complete  in  the  single 
lobule,  one  half  of  which  is  represented  in  Figure  395.  The  two  kinds 
of  cells  are  very  irregularly  arranged  in  the  resulting  germinal  epithe- 
lium, but  when  growth  has  progressed  to  the  stage  seen  in  Figure  396, 
which  represents  a  small  portion  of  a  section  of  one  of  the  lobules,  it  can 

be  seen  that  the  reproduc- 
tive cells  all  have  a  proxi- 
mal position  and  the  nurse 
cells  have  a  distal  position, 
forming  a  single  row  on 
the  edge.  A  rather  re- 
markable change  begins  to 
take  place  now,  as  is  indi- 
cated in  this  figure  by  the 
withdrawal  of  two  of  the 
nurse  cells  proximally. 
This  change  consists  in  a 
migration  of  all  of  the 
nurse  cells  to  form  a  single  proximal  layer  on  the  capsule  wall,  leaving 
the  many  layered  reproductive  epithelium  on  the,  distal  surface. 


FIG.  395.  —  Section  of  one  half  of  an  older  lobule  than 
seen  in  Fig.  394.  n.c.,  nurse  cells ;  ogn.,  spermatogonia. 
X  1000. 


438 


HISTOLOGY 


The  next  stage  (Fig.  397)  shows  spermatogonial  divisions  and  at  the 
same  time  the  nurse  cells  can  be  seen  in  the  midst  of  then*  migration. 


FIG.  396.  —  Part  of  a  section  of  an  older 
sperm  lobule  of  Raja  ocellata  than  that 
seen  in  Fig.  395.  Nurse  cells  (n.c.)  lie 
in  a  distal  position.  Spermatogonia 
numerous.  X  1000. 


FIG.  397.  —  Part  of  an  older  sperm  lobule  of 
Raja  ocellata  than  seen  in  Fig.  396.  Nurse 
cells  (n.c.)  migrating  to  a  proximal  posi- 
tion. Spermatogonia  dividing,  x  1000. 


In  Figure  398  this  migration  is  finished  and  the  reproductive  cells 
(spermatogonia)  have  just   finished  their  multiplication  divisions.     At 
S^TV-,..^:'  this  time  they  are   from  six  to 

eight  rows  deep  and  must  num- 
ber in  the  neighborhood  of  8000 
spermatogonia  for  each  lobule. 

The  spermatogonia  now  go 
through  the  synizesis  stage,  or 
contraction  stage,  which  is  rep- 
resented in  Figure  398.  After 
this  they  expand  the  chromatin 
thread  and  appear  as  the  cells 
on  the  left  of  Figure  399.  They 
are  now  known  as  the  primary 
spermatocytes.  In  these  it  can 
be  seen  that  the  chromatin  shows 
ring-like  structures  at  places. 
These  are  the  forming  tetrads 
spoken  of  in  the  general  discus- 
sion, and  they  soon  become 

FIG.  398-—  Part  of  an  older  sperm  lobule  of  Raja  _inQ_iv     oocpmhlprl      in     a     dense 

ocellata  than  seen  in  Fig.  397.    Nurse  cells  now  ClOSCly   ^  assemble  ISC, 

all  rest  on  proximal  surface.     Spermatogonia  in  equatorial    plate,    which   then    dl- 

synizesis  or  contraction  stage.     X  100.  ^^    by   a    pulling    of    tne    rhlg- 

shaped  chromosomes  into  two  halves.     This  may  be  seen  in  the  right- 
hand  portion  of  Figure  399. 


MALE  REPRODUCTIVE  CELLS 


439 


The  cells  formed  by  this  division  are  the  secondary  spermatocytes; 

they  are  shown  in  Figure  400.    They  are  sometimes  hard  to  distin- 
guish  from   the  early  sperma- 

tids.     It  can  be  seen  here  that, 

for  almost  the  first   time,  the 

germinal   epithelium    is    being 

divided    into    the    sperm   col- 
umns.   Two  of  these  are  to  be 

seen  in  the  figure,  and  at  the 

base  of  each  is  to  be  seen  a 

single,  larger  nurse  cell,  some- 
times   called    in    the    male    a 

Sertoli   cell.    The    cell    bodies 

cannot    be    separated   by  any 

line    of   demarcation,  and  the 

nuclei  are  large  and  lie  flat  on 

the  basement  membrane. 

The  reproductive  cells  soon 

divide    again,    first   expanding 

their  chromatin  reticulum  into 

a  stage  which  is  not  figured. 

The     divisions    of    secondary 

spermatocytes  into  spermatids 

are  seen  in  Figure  401.    They  are  far  smaller  than  the  previous  division 

figures  of  the  primary  sper- 
matocyte  into  two  secondary 
spermatocytes.  Moore  de- 
scribes the  dividing  chromo- 
somes of  this  second  division 
as  separating  globules,  rather 
than  rings  which  break  in 
halves.  The  cell  bodies  are 
rounded  and  have  become 
more  separated  from  one 


FIG.  399.  —  Tetrad  formation  and  first  reduction 
divisions  in  Raja  ocellata.     X  1000. 


No    changes    have 
in    the    position   or 


another, 
occurred 

appearance  of  the  nurse  cells. 
The  lower  part  of  figure  401 
shows  some  of  these  divisions 
in  the  latter  stages.  To  the 
right  are  seen  the  results  of 
these  divisions,  the  youngest  spermatids.  The  development  of  these 
into  almost  mature  spermatozoa  is  shown  in  the  next  four  figures. 


FIG.  400. — Second  spermatocytes  of  Raja  ocellata. 
Nurse  cells  on  basement  membrane.  The  repro- 
ductive cells  begin  to  show  a  tendency  toward  the 
formation  of  sperm  columns.  X  1000. 


440 


HISTOLOGY 


m 


FIG.  401.  —  Second  reduction  divisions  by  the  sec 
ondary  spermatocytes  of  Raja  oscellata.    x  1000. 


Figure  402  shows  two  sperm 
columns,  each  extending  dis- 
tally from  the  nurse  cell  which 
belongs  to  it.  It  can  here  be 
noticed  that  each  column  is 
cup-shaped,  and  that  its  sec- 
tion forms  an  elongated  loop 
with  the  bend  placed  against 
the  nurse  cell.  The  nucleus 
has  moved  to  the  proximal 

/rafcyUft*?^  ^vwrv^Jfei* ^-^  .x.  f $$'V)r>X    end  of  the  cell  in   each  sper- 
;'^^J|f -'.^ii  .b^fj^  •'.  G^  :^:%^v-<^    matid,  and  a  filament,  the  devel- 
:f«^^^:i^,-?.Ufl»».-j^54~7f?Hfej          oping  tail,  passes  distally  from 

the  nucleus  out  of  the  end  of 
the  cell. 

Figure  403  shows  a  consid- 
erable advance  on  the  last 

The  cells  have  all  moved  proximally,  in  the  three  sperm 
columns  shown,  and  have  gathered  closely  in  the  loop  by  the  basal 
nurse  cells.  These  latter  have  remained  flat.  Each  spermatid  has 
become  much  elongated  and  so  has  the  nucleus,  which  is  pointed 
distally  and  has  a  ring-like 
enlargement  at  its  flat  distal 
end.  The  cell  outlines  are 
poorly  shown,  but  where  cut 
transversely  as  shown  in  the 
upper  part  of  the  figure 
their  outline  is  quite  ap- 
parent with  a  dot,  repre- 
senting the  tail,  in  the 
center  of  each. 

The  change  from  this  last 
stage  to  that  seen  in  Figure 
404  is  somewhat  wide,  but 
space  forbids  a  closer  com- 
parison. In  the  latter  stage 
the  half-developed  sperma- 
tozoa have  greatly  length- 
ened and  have  all  moved 
down  so  that  their  sharp 

heads     are      buried     in     the  FIG.  402.  —  Two  nurse  cells  and  two  sperm  columns 

nurse    cells.      This    head,    it  comP°sed  of  y°ung :  spermatids ,in  Raja  ocellata.     The 

'  tails   and   end-knobs   are   beginning   to   form.      Cell 

Will     be     Seen,    is    the    elon-  walls  are  still  complete.     X  1000. 


MALE  REPRODUCTIVE    CELLS 


441 


gated  nucleus  and  its  posterior  end  is  much  curled.    The  cell  body 

extends,  as  a  middle  piece,  back  from  the  distal  end  of  the  head  for 

about  the   same   length  that 

the   head  is.     It  invests   the 

axial  filament  of  the  tail,  and 

distally  the  tail  emerges  and, 

except  for  the  fin  which    is 

not  shown,  is  free  for  the  rest 

of  its  course.    The  nurse-cell 

nuclei  at  this  time  have  moved 

up  from  the  flat  position  which 

they  occupied  before  and  lie 

alongside  of  the  heads  of  the 

spermatids  which  are  attached 

to  them.    They  are  rounder 

and  fuller  than  before. 

The  last  stage  to  describe 
is  shown  in  Figure  405.    This 
shows    how    the     spermatids 
have     shortened 
straightened    the 

been       gathered       mtO       Close    FIG.  403.  —  Spermatids  of  Raja  ocellata.    The  nuclei 

are  lengthening  and  the  tails  are  forming.  Above 
are  a  number  of  tails  cut  transversely  and  showing 
the  outlines  of  the  cells  whose  centers  they  occupy. 
These  reproductive  cells  form  two  sperm  columns, 
which  are  placed  opposite  to  two  large  nurse  cells 


n  length, 
head,  and 
into  close 
points  are 


on  the  basement  membrane.     X  1000. 


bundles.     Their 
still  buried  in  the  nurse  cells, 
and  the  nuclei  of  these  later 
cells  are  now  large,  full,  and 
have     well-developed      chro- 

matin  structure,  as  well  as  a  larger  nucleolus  than  before.  A  rather 
remarkable  vacuole  appears  at  this  time  in  the  cytoplasm  of  the  nurse 
cell  and  is  filled  with  a  dense  mass  that  stains  gray  in  iron  haema- 
toxylin.  This  mass  rests  against  the  distal  end  of  the  nurse-cell  nucleus 
and  is  probably  connected  in  some  unknown  way  with  the  nutritive 
processes  which  are  going  on  at  this  time.  Other  smaller  granules  appear 
in  the  proximal  cytoplasm.  It  should  be  noticed  in  both  this  stage  and 
in  the  last  that  the  head  of  the  spermatid  is  connected  with  the  basement 
by  a  transparent  strand  of  cytoplasm  which  passes  through  the  body  of 
the  nurse  cell.  The  spermatozoa  are  now  nearly  ripe,  and  it  is  in  this 
condition  that  they  separate  from  the  nurse  cell  and  pass  out  of  the  lobe 
with  the  rupture  of  the  lobule  at  its  surface.  The  nurse  cells  remain 
behind,  showing  more  individuality  at  this  time  than  at  any  other  period 
of  their  lives.  Three  of  them  are  shown  by  Figure  406  rising  from  the 
inner  wall  of  a  recently  ruptured  follicle. 

The  above  description  of  the  production  of  spermatozoa  furnishes 


442 


HISTOLOGY 


a  demonstration  of  the  more  ordinary  features  of  this  process  in  the  ani- 
mals. The  more  superficial  features  are  discussed;  but  as  in  the  majority 
of  cases  among  animals  the  chromatin  does  not  show  all  the  changes  it 
probably  goes  through,  or  allow  us  to  explain  the  probable  meaning  of 
such  changes  as  we  can  see.  Dr.  H.  E.  Jordan  has  very  kindly  per- 
mitted us  to  use  the  following  account  he  has  written,  and  the  figures  he 
has  drawn  of  his  studies  on  the  spermatogenesis  of  Aplopus  Mayeri,  a 
giant  "walking  stick"  of  the  Florida  Keys,  to  demonstrate  these  features. 


FIG.  404.  —  Half  developed  spermatozoa  of  Raja  ocellata.  Middle  pieces  are  visible.  The 
members  of  each  sperm  column  have  been  drawn  in  or  have  moved  in  to  bury  their  tips 
in  the  nurse  cells.  X  1000. 

The  accessory  chromosome  and  its  relation  to  the  phenomenon  of 
sex.1  — The  accessory  chromosome  (so  named  by  McClung,  '02)  was 
first  reported  by  Henking  in  a  paper  on  the  spermatogenesis  of  Pyr- 
rhocoris  apterus  (Hemipter)  published  in  1890.  Henking  here  noticed 
that  in  one  of  the  spermatocyte  divisions  one  chromosome  did  not  divide, 
thus  giving  rise  to  two  kinds  of  spermatozoa,  one  group  with  and  the 
'other  without  the  odd  element.  McClung  ('oo-'o2)  studied  a  number 
of  forms  among  the  Locustidae  and  Acrididae,  and  reported  uniform 
results  in  regard  to  the  presence  and  behavior  of  the  accessory  chromo- 

1  Written  by  Dr.  H.  E.  Jordan  of  the  University  of  Virginia. 


MALE  REPRODUCTIVE   CELLS 


443 


some.  In  the  Orthoptera  he 
was  able  to  trace  this  chro- 
mosome back  to  the  sperma- 
togonial  rest  stage.  McClung 
was  the  first  to  suggest  a  pos- 
sible casual  connection  between 
the  dimorphism  of  sex  and  the 
observed  dimorphism  of  the  sper- 
matozoa. His  conclusions  were 
drawn  from  his  observations  on 
the  maturation  phenomena  of 
the  Insecta,  and  the  fact  that  sex 
appears  to  be  the  only  character 
that  divides  the  individuals  of  a 
species  into  two  approximately 
equal  groups. 

Recently  there  has  been 
great  activity  in  the  study  of  the 
accessory  chromosome  and  in  a 
search  for  this  element  in  the 
insects.  It  has  been  very  thor- 
oughly studied  by  Wilson  ('05- 
'06)  in  several  of  the  Hemiptera 
heteroptera  (Anasa  tristis,  Pro- 
tenor  belfragei,  Alydus  pilosulus, 
Harmostes  reflexulus,  Archimerus 
calcarator,  and  Banasa  calva). 
Here  the  accessory  chromosome 

is  associated  with  a  pair  of  small  chromosomes  which  behave  differ- 
ently from  the  ordinary  chromosomes  (they  remain  condensed  in  the 
growth  stage)  and  are  called  by  Wilson  "microchromosomes."  Miss 

Stevens  ('06)  has  reported  an 
accessory  chromosome  in  Aphro- 
phora  quadrangular  is,  one  of 
the  Hemiptera  homoptera  (here 
also  associated  with  microchro- 
mosomes) and  in  certain  of  the 
Coleoptera.  An  odd  chromo- 
some very  similar  in  behavior 
to  the  accessory  of  orthoptera 
FIG.  406. — Three  nurse  cells  on  the  basement  has  been  reported  by  Berry  ('02) 


FIG.  405.  —  Fully  developed  spermatozoa  of  Raja 
ocellata.  The  heads  and  middle  pieces  have 
shortened  and  the  sperm  columns  have  become 
compacted  into  regular  bundles.  Peculiar  bod- 
ies have  developed  on  the  distal  end  of  each 
nurse  cell  nucleus.  X  1000. 


membrane  of  a  sperm  lobule  of  Raja   ocellata. 
The  spermatozoa  have  been  discharged  from  the 

lobule,    x  1000. 


• 
m 


('05)    in   Scolopendra;   however, 


444  HISTOLOGY 

the  typical  accessory  chromosome  (unassociated  with  micro-  or  idio- 
chromosomes)  is  found  only  among  the  Orthoptera,  where  it  has  been 
described  by  Sutton  ('oo-'o2)  in  Brachystola  magna;  by  de  Sinety  ('01) 
in  one  of  the  Acridities,  and  several  of  the  Phasmida;  by  Baumgart- 
ner  ('04)  in  Gryllus  domesticus;  by  Stevens  ('05)  in  Stenopelmatus  and 
Blatella  germanica;  and  by  Otte  ('06)  in  Locusta  viridissima.  Moore 
and  Robinson  ('05)  claim  that  in  Periplaneta  Americana  the  odd  chro- 
mosome is  merely  a  plasmosome,  dissolving  before  each  division  and 
re-forming  after  it. 

A  different  terminology  has  been  employed  by  various  writers  to 
designate  the  accessory  chromosome  of  McClung.  Miss  Stevens  calls 
it  the  "odd  chromosome";  Montgomery  formerly  used  the  term  "chro- 
matin  nucleolus";  de  Sinety  designates  it  the  "chromosome  speciale"; 
and  Wilson  names  it  the  "  heterotropic  chromosome,"  where  it  appears 
in  the  Hemipters. 

Much  theory  and  speculation  has  arisen  in  a  regard  to  the  accessory 
chromosome  and  its  supposed  connection  with  the  inheritance  and  de- 
termination of  sex.  Castle  ('03)  has  developed  a  theory  of  sex  in  which 
he  applies  Mendel's  principle  of  segregation  to  sex  phenomena.  He 
shows  that  sex  production  may  be  explained  as  the  result  of  a  Mendel  ian 
segregation,  transmission,  and'  dominance  of  sexual  characters.  The 
theory  has  recently  been  more  fully  elaborated  by  Wilson  ('06)  and 
extended  to  apply  to  cases  where  either  of  the  three  forms  of  hetero- 
chromosomes  (Montgomery)  prevail:  microchromosomes  (small  chromo- 
somes); idiochromosomes  (a  pair  of  unequal  chromosomes);  or 
a  heterotropic  chromosome.  The  common  character  of  heterochro- 
mosomes  is  their  compact  nature  and  deep  staining  reaction  during  the 
various  stages  of  growth  and  maturation  when  the  ordinary  chromo- 
somes pass  into  the  nuclear  reticulum.  The  theoretical  discussion  of 
the  observations  on  the  accessory  chromosome  and  the  far-reaching 
conclusions  which  may  be  drawn  from  the  observed  facts  are  better 
understood  after  a  brief  presentation  of  a  concrete  example.  To  this 
end  Aplopus  Mayeri  (Phasmid),  the  giant  walking-stick  insect  of 
Loggerhead  Key,  Florida,  serves  the  purposes  admirably.  Here  the 
accessory  chromosome  can  be  traced  from  its  first  origin  in  the  secondary 
spermatogonial  cells  through  the  entire  history  of  spermatogenesis  into 
the  nucleus  of  a  half  of  the  spermatids,  where  it  finally  disappears  during 
the  time  that  these  undergo  metamorphoses  into  spermatozoa.  Except 
for  the  number  of  chromosomes,  the  facts,  as  they  here  obtain,  agree  in 
essential  points  with  Wilson's  latest  report  (after  an  extensive  compara- 
tive study  of  smear,  unfixed,  and  fixed  and  stained  preparations)  for 
Anas  a  tristis. 

The  primary  spermatogonial  cells  (Fig.  407,  i)  have  a  resting  nucleus 


MALE  REPRODUCTIVE    CELLS 


445 


with  pale  reticulum  and  an  occasional  karyosome.  Nothing  resem- 
bling an  accessory  chromosome  or  even  a  plasmosome  can  be  detected. 
The  cells  divide  mitotically  (occasional  amitotic  divisions  also  occur) 


FIG.  407.  —  Male  reproductive  cells  of  Aplopus  Mayeri.  i,  resting  primary  spermatogonium ; 
2,  chromosome  count  of  dividing  primary  spermatogonium;  3,  resting  secondary  spermato- 
gonium showing  accessory  chromosome;  4,  equatorial  view  of  the  35  chromosomes  of  sec- 
ondary spermatogonial  division;  5,  equatorial  view  of  the  36  chromosomes  of  a  dividing 
egg-follicle  cell  of  female,  x  1400.  (Drawn  by  H.  E.  JORDAN.) 

in  the  ordinary  homeotypic  fashion  and  the  equatorial  plate  yields  a 
chromosome  count  of  35  (Fig.  407,  2);  none  of  these  can  be  definitely 
identified  as  the  future  accessory.  During  the  resting  stage  of  the 
secondary  spermatogonial  cells  (Fig.  407,  3),  the  accessory  chromosome 
appears  as  a  deep  staining  body  amid  the  pale  reticulum,  situated  close 
to  the  nuclear  wall.  Sometimes  there  appears  a  suggestion  of  a  bipartite 
structure  and  occasionally  even  of  a  compact  skein  structure.  The  cells 
always  divide  by  homeotypic  mitosis,  and  the  equatorial  plates  again 
contain  35  enlarged  chromosomes  (Fig.  407,  4).  There  are  six  or 
seven  generations  of  secondary  spermatogonia  and  only  during  the 
telophase  of  the  final  spermatogonial  division  (prophase  of  primary 
spermatocytes)  does  the  accessory  chromosome  retain  its  compact  form 
and  deep  staining  capacity  (Fig.  408,  6)  among  the  ordinary,  pale 
mossy  chromosomes,  and  thus  passes  over,  in  original  form,  into  the 


FlG.  408. — Aplopus  Mayeri.  6,  telophase  of  final  spermatogonial  division;  7,  resting  stage  of 
pre-synaptic  period ;  8,  formation  of  chromatin  lattice  and  lengthening  of  accessory  chromo- 
some; 9,  spireme  loops  and  accessory  chromosome  of  pre-synaptic  growth  period;  10,  open- 
ing of  chromatin  loops  before  synapsis;  n,  synapsis  of  chromosomes.  X  1400.  (Drawn 
by  H.  E.  JORDAN.) 


resting  or  pre-synaptic  stage  of  the  growth  period  of  the  primary  sper- 
matocyte  (Fig.  408,  7).  Figure  407,  5,  shows  a  metaphase  group  of  a 
dividing  follicle  cell  of  the  ovary  with  36  chromosomes.  The  primary 


446  HISTOLOGY 

spermatocyte  now  passes  through  a  growth  period  during  which  it  in- 
creases somewhat  in  size  (Figs.  7-15). 

The  growth  period  presents  various  stages  of  intense  activity  and  great 
protoplasmic  alterations.  After  a  brief  resting  stage  (Fig.  408,  7)  it  enters 
upon  a  pre-synaptic  phase,  during  which  the  nuclear  reticulum  becomes 
slightly  chromatic  and  disposes  itself  into  a  lattice-work  arrangement 
(Fig.  408,  8).  At  the  same  time  the  accessory  chromosome  lengthens 
out  into  a  club-shaped  mass  extending  through  almost  the  entire  diameter 
of  the  nucleus  and  becoming  attached  at  its  lesser  end  to  the  nuclear 
spireme.  The  spireme  now  breaks  up  into  a  number  of  segments 
(approximately  34)  and  these  form  loops  at  one  pole  of  the  nucleus  (Fig. 
408,  9).  This  represents  synizesis.  Subsequently  these  loops  open 
up  and  one  end  becomes  free  (Fig.  10).  The  segments  now  unite  in 
pairs  by  their  free  ends  (Fig.  n)  to  form  half  the  number  of  original 
loops.  This  stage  is  synapsis  and,  according  to  Montgomery  and  others, 
represents  a  pairing  of  homologous  paternal  and  maternal  chromosomes. 


FlG.  409. — Aplopus  Mayeri.  Four  stages  in  postsynapsis,  showing  postsynaptic  reticulum 
and  closing  up  of  the  elongate  and  ring-shaped  accessory  chromosome.  X  1400.  (Drawn 
by  H.  E.  JORDAN.) 

The  large  loops  are  the  bivalent  postsynaptic  chromosomes.  This 
end-to-end  union  is  an  example  of  telosynapsis.  Meanwhile  the  acces- 
sory chromosome  assumes  a  position  to  one  side  of  and  usually  beneath 
the  loops.  Its  longitudinal  split  opens  up  more  or  less  completely  during 
this  stage. 

During  subsequent  postsynaptic  stages  the  chromosomes  again 
arrange  themselves  into  a  reticulum  (frequently  giving  indications  of  a 
longitudinal  split)  in  the  shape  of  a  lattice-work.  The  accessory  chro- 
mosome, meanwhile,  closes  up  and  shortens  down  into  a  compact,  deep 
staining  body  closely  applied  to  the  nuclear  wall  (Fig.  409,  12-15). 
During  the  ensuing  prophases  (Fig.  410,  16-18)  the  spireme  becomes 
split  into  a  number  of  segments  (17),  each  of  which  presently  undergoes 
first  a  longitudinal,  and  secondarily  a  transverse  fission,  to  form  typical 
tetrads  of  various  forms  and  sizes.  While  the  ordinary  chromosomes 
are  at  this  stage,  and  still  pale  staining,  the  accessory  chromosome  is 
readily  distinguishable  as  a  body  of  deep  staining  capacity  and  sharp 
contour.  It  varies  much  in  form,  being  bipartite,  quadripartite,  or 
U-shaped.  A  later  prophase  (Fig.  410,  19)  shows  all  the  chromosomes 


MALE  REPRODUCTIVE    CELLS 


447 


deeply  stained  (many  in  typical  tetrad  form),  among  which  the  acces- 
sory is  only  occasionally  recognizable  where  it  has  a  large   U-shaped 


FIG.  410. — Aplopus  Mayeri.  16,  17,  and  18,  three  early  prophases  of  the  first  reduction  divi- 
sion, all  chromosomes  except  the  accessory  are  still  light  staining;  19,  prophase  in  which 
the  chromosomes  are  forming  tetrads  and  the  accessory  is  not  recognizable  because  all  the 
chromosomes  stain  dark;  20,  equatorial  plates  of  four  primary  spermatocytes  about  to  per- 
form first  reduction  div'sion.  Each  shows  18  chromosomes,  one  of  which  is  the  accessory. 
X  1400.  (Drawn  by  H.  E.  JORDAN.) 

form.  Figure  20  shows  equatorial  plates  of  four  contiguous  primary 
spermatocyte  cells  during  the  metaphase  of  the  first  maturation  mitosis. 
Each  of  the  cells  has  18  chromosomes,  one  of  which  is  the  acces- 


FlG.  411.  —  Aplopus  Mayeri.  Four  stages  in  the  first  reduction  division,  showing  how,  in  the 
operation,  the  accessory  chromosome  passes  undivided  to  one  of  the  daughter  cells  or  sperm. 
ac.,  accessory  chromosome.  X  1400.  (Drawn  by  H.  E.  JORDAN.)  » 


sory,  not  recognizable,  however,  among   the    ordinary  chromosomes. 
It  is  one  of  the  large  eccentric  bodies;   other  plates  show  the  unmis- 


448 


HISTOLOGY 


takable  U-shaped  accessory  chromosome  in  this  position.    In  Figure  411, 
21,  are  seen  the  ordinary  chromosomes  in  the  spindle  at  metaphase. 

The  first  maturation  di- 
vision is  undoubtedly  trans- 
verse, separating  whole 
chromosomes,  and  is  there- 
fore a  true  reduction  divi- 
sion. The  second  maturation 
mitosis  is  the  equation  divi- 
sion (this  being  sometimes 

FlG.  412.  —  Aplopus  Mayeri.   25  and  26,  res  ting  stages  consummated        precociously 

of  second  spermatocyte  with  accessory  chromosome;  •       ^      tPlnnV,flcp   r>f    trip    first 
27,  a  pair  of  sister  spermatocytes  of  second  order, 

one  of  which  has,  and  the  other  consequently  has  mitosis),   both    for    the    Ordl- 

SV^ajSSS  }cllromosome-  x  I4°°-  (Drawn  nary  and  accessory  chromo- 
somes. In  the  first  reduc- 
tion division  the  accessory  usually  assumes  a  position  on  the  spindle  in 
advance  and  to  one  side  of 
the  other  chromosomes  (Figs. 
22-23).  In  the  later  telophase 
it  almost  invariably  splits  into 
two  portions,  the  result  of  a 
separation  of  the  two  arms 
of  the  U  at  the  bend  (Fig.  24). 
Obviously,  since  the  accessory 
chromosome  passes  undivided 
to  one  of  the  poles  of  the  first 
maturation  spindle,  the  result- 
ing daughter  cells  (secondary 
spermatocytes)  are  of  two 
classes,  i.e.  those  with  the 
accessory  and  those  lacking 
it  (Fig.  27).  A  brief  resting 
stage  ensues,  during  which 
the  nuclear  wall  is  recon- 
structed (Fig.  25)  and  the 
mass  of  chromosomes  disen- 
tangles and  diffuses  into  a 
pale  reticulum. 

Meanwhile    the    accessory 
chromosome     has     remained 
separate    from    the    ordinary 
chromosomes  and  has  retained  its  characteristic   form  and  position 
in  the   nucleus.     Very  soon  the  reticulum  passes  through  the  regular 


FlG.  413.  —  A  plopus  Mayeri.  2  8,  2  9,  30,  three  stages 
in  the  second  reduction  division  of  the  spermato- 
cytes of  the  second  order  to  form  spermatids;  28 
and  29  show  equation  divisions  of  the  accessory 
chromosome;  31,  equatorial  plates  of  four  second- 
ary spermatocytes ;  two  possess  the  accessory  chro- 
mosome while  the  other  two  do  not ;  32 ,  telophase 
of  second  reduction  division ;  accessory  chromosome 
not  distinguishable  among  other  dark  staining 
chromosomes.  X  1400.  (Drawn  by  H.  E.  JORDAN.) 


MALE   REPRODUCTIVE    CELLS 


449 


prophase  stages  of  fine,  coarse,  and  segmented  spireme  (Fig.  412, 
26-27).  The  stages  of  the  regular  homeotypic  mitosis  follow  (Fig. 
413,  28-30).  A  pair  of  chromosomes,  larger  than  their  fellows,  lags 
behind  in  its  entrance  into  the  spindle  and  its  passage  to  the  poles  in 
some  of  the  cells.  This  pair,  seen  in  Figures  28-29,  are  the  products  of 
an  equation  division  of  the  accessory  chromosome.  Obviously  again, 
equatorial  plates  of  different  spindles  ought  to  show  a  chromosome 
count  varying  between  17  and  18,  the  latter  count  including  the  accessory 
chromosome.  Figure  31  shows  the  metaphase  groups  of  chromosomes 
of  four  contiguous,  secondary  spermatocytes.  These  are  pairs  of 
daughter  cells  of  a  pair  of  primary  spermatocyte  mother  cells.  The 
chromosome  count  alternates  between  18  and  17  among  the  groups. 
The  first  and  third  are  seen  to  contain  a  large  U-shaped  chromosome 
at  the  periphery  of  the  complex.  This  is  the  accessory  chromosome. 
Telophase  stages  of  this 
mitosis  (Figs.  32-33)  do 
not  reveal  the  accessory 
as  a  distinct  body  within 
the  general  chromatin 
mass;  but  as  soon  as  the 
nuclear  wall  of  the  result- 
ing spermatid  is  recon- 
structed, the  accessory,  of 
typical  form  and  location, 
again  presents  itself  in  the 
pale  staining  reticulum 
(Fig.  414,  34).  In  Figure 
35  is  shown  a  spermatid 
in  the  first  stages  of  meta- 
morphosis to  become  a 
spermatozoon.  It  contains 
the  accessory  as  a  chro- 
matic spherical  eccentric  body.  A  middle  piece  has  grown  out  from 
a  centrosome-like  granule  applied  to  the  nuclear  wall,  and  terminates 
in  a  long  slender  filament  or  tail  about  which  later  develops  a  cyto- 
plasmic  spiral  fin.  Later  stages  in  the  metamorphosis  are  shown  in 
Figures  36  and  37,  each  figure  representing  a  pair  of  spermatids  with 
and  without  the  accessory,  respectively.  Finally  the  accessory  disap- 
pears in  the  head  of  the  ripening  spermatozoon. 

The  observed  facts,  then,  that  may  be  employed  in  forming  a  theory 
concerning  the  role  of  the  accessory  chromosome  in  the  determination 
of  sex  as  it  has  been  developed  by  McClung,  Castle,  and  most  fully 
elaborated  and  most  widely  applied  by  Wilson  are  these:  (i)  an  odd 


FIG.  414.  —  Aplopus  Mayeri.  33,  telophase  of  second  re- 
duction; no  accessory  chromosome ;  34,  spermatid,  and 
35,  spermatozoon,  with  accessory  chromosome;  36  and 
37,  a  younger  and  an  older  pair  of  spermatozoa,  one  of 
each  pair  with  and  the  other  without  an  accessory  chro- 
mosome. X  1400.  (Drawn  by  H.  E.  JORDAN.) 


45O  HISTOLOGY 

number  of  chromosomes  (35)  in  the  spermatozoon;  (2)  an  even  number 
of  chromosomes  (36)  in  the  follicle  cells  of  the  ovary  (it  has  been  proved 
in  various  forms  of  insects  that  the  number  of  somatic  chromosomes  is 
the  same  as  that  of  the  presynaptic  germ  cells);  (3)  the  appearance  of 
a  chromatin  nucleolus  in  the  spermatogonial  cells  which  persists  during 
the  last  mitosis  preceding  the  spermatocyte  phase  and  by  its  staining 
reaction  and  behavior  during  maturation  proves  itself  a  true  chromosome 
(accessory  chromosome);  (4)  the  absence  of  a  mate  with  which  the  acces- 
sory may  pair  during  synapsis;  (5)  failure  of  the  accessory  to  divide  in 
the  first  maturation  mitosis ;  (6)  a  resulting  dimorphism  of  spermatozoa, 
consisting  in  the  presence  (18  chromosomes)  and  absence  (17  chromo- 
somes) of  the  accessory;  (7)  a  reduction  in  the  number  of  chromosomes, 
during  the  maturation  of  the  egg,  to  half  the  somatic  number  (18  chro- 
mosomes). 

From  the  above  facts  it  follows  that,  if  a  mature  egg  possessing  18 
chromosomes  is  fertilized  by  a  spermatozoon  with  18  chromosomes,  an 
organism  results  which  has  36  somatic  chromosomes,  and  this  is  known 
by  observation  to  be  a  female.  Again  if  such  an  egg  be  fertilized  by 
a  spermatozoon  with  17  chromosomes,  an  organism  with  35  chromosomes 
results  and  this  is  known  to  be  a  male.  The  presence  of  an  additional 
chromosome  (the  accessory)  thus  distinguishes  the  female  from  the  male 
cell  from  the  chromosome  standpoint ;  hence  the  accessory  chromosome 
appears  to  have  some  connection  with  the  kind  of  sex  that  is  to  be  pro- 
duced. The  final  test  of  the  theory  that  the  accessory  chromosome  is 
a  sex-determinant  lies  in  direct  experiment  with  that  element  in  fertili- 
zation, the  difficulties  of  which  have  thus  far  remained  insuperable. 
Meanwhile,  however  far  the  theory  may  accord  with  fact,  it  remains  a 
question,  as  Bateson  has  suggested,  whether  the  accessory  body  may  not 
be  merely  associated  with  the  cause  of  sex;  or,  as  Wilson  suggests,  the 
morphological  expression  of  a  hidden  physiological  cause.  The  most 
that  may  perhaps  be  claimed  for  the  accessory,  in  relation  to  sex  pro- 
duction, is  that  it  represents  sex  characters,  and  this  is  further  based  on 
the  assumption  that  the  chromosomes  really  are  the  vehicles  that  carry 
the  hereditary  characters. 

As  alternative  theories,  Wilson  suggests:  (i)  that  the  heterochro- 
mosomes  may  merely  transmit  sex  characters,  and  that  sex  itself  is 
determined  by  protoplasmic  conditions  external  to  the  chromosomes; 
(2)  that  the  accessory  may  be  a  sex  determinant  only  by  virtue  of  a  dif- 
ference in  activity  or  amount  of  chromatin.  Paulmier  and  Montgomery 
believe  that  the  accessory  is  a  degenerating  chromosome  and  that  its 
presence  represents  a  stage  in  the  evolution  of  a  species  from  a  higher  to 
a  lower  chromosome  number.  They  further  believe,  with  Wilson,  that 
it  represents  the  persisting  larger  member  of  a  pair  of  idiochromosomes, 


MALE  REPRODUCTIVE    CELLS 


451 


the  smaller  member  of  which  has  disappeared.  This  suggestion  is  help- 
ful in  the  further  development  of  the  theory  of  sex  determination  by 
the  accessory  chromosome.  Again  the  elaboration  and  extension  of  this 
point  has  been  made  by  Wilson.  It  was  originally  believed  byMcClung 
and  Paulmier  that,  since  the  male  germ  cell  carried  the  accessory  body, 
eggs  fertilized  by  spermatozoa  containing  this  element  should  produce 
males.  This  conclusion  was  based  upon  an  erroneous  observation  that 
the  male  somatic  cells  had  more  chromosomes  than  the  female.  Wilson 
has  demonstrated  in  the  case  of  Anasa  tristis  that  exactly  the  reverse  is 
true,  and  the  same  fact  holds  for  Aplopus  and  other  forms  of  insects. 

Applying  the  same  line  of  argument  to  Aplopus,  and  making  similar 
assumptions  to  those  suggested  by  Castle  for  an  interpretation  of  sex  along 
Mendelian  lines:  (i)  two  kinds  of  eggs  (male  and  female)  as  also  two 
kinds  of  spermatozoa  which  have  been  actually  frequently  observed; 
(2)  selective  fertilization  or  infertility  of  gametic  unions  of  like  sex 
chromosomes,  i.e.  an  egg  with  a  female  determinant  must  be  fertilized 
by  a  sperm  with  a  male  determinant, 
and  vice  versa;  (3)  the  dominance  of 
fcmaleness ;  also  the  further  assump- 
tion suggested  by  Wilson;  (4)  that 
the  accessory  chromosome  repre- 
sents the  larger  member  of  a  pair  of 
idiochromosomes,  the  smaller  mem- 
ber of  the  pair  having  been  lost — it 
is  then  seen  that,  since  a  mature 
egg  of  1 8  chromosomes  fertilized  by 
a  spermatozoon  lacking  the  acces- 
sory gives  origin  to  a  male,  the  male 
determinant  must  have  been  intro- 
duced by  the  egg  and  that  the  miss- 
ing mate  to  the  accessory  was  a 
female  determinant.  Furthermore, 
since  when  such  an  egg  is  fertilized 
by  a  spermatozoon  possessing  the 
accessory  chromosomes  a  female 
arises,  the  egg  must  have  contributed 
a  female  determinant  to  which  the 
accessory  or  male  determinant  is  re- 
cessive ;  it  thus  also  appears  that  the 
accessory  chromosome  alternates  be- 
tween the  two  sexes  in  successive  generations,  i.e.  the  male  contributes  the 
accessory  to  the  female  in  the  production  of  the  female,  and  the  female  in 
the  ensuing  generation  contributes  the  accessory  to  the  male  in  the  pro- 


FIG.  415.  —  Diagram  to  illustrate  the  part 
played  by  the  accessory  chromosome  in  the 
determination  of  sex  during  maturation  and 
conjugation.  (After  WILSON.) 


452  HISTOLOGY 

duction  of  a  male.  This  fact  is  admirably  illustrated  in  the  above 
diagram  by  Wilson  (Fig.  415).  Modifying  Wilson's  formula  for  sex 
production,  to  cover  the  case  of  Aplopus,  the  whole  theory  with  facts  and 
assumptions  may  be  succinctly  stated  as  follows :  — 

I.     ?   Egg  (18  chromosomes)   +  (  $  )  sperm  (18  chromosomes)  = 
$  (  $  )  female  (36  chromosomes). 

II.  $  Egg  (18  chromosomes)  +  (o)  sperm  (17  chromosomes)  = 
(  $ )  (o)  male  (35  chromosomes). 

Technic.  —  Owing  to  the  lack  of  yolk,  and  to  the  uniform  protoplas- 
mic texture  of  the  specific  male  reproductive  tissues,  the  technic  has  been 
easy,  requiring  only  great  care  and  exactness  and  a  reasonable  apprecia- 
tion of  the  element  of  luck,  represented  by  the  unknown,  and  sometimes 
unmeasurable,  factors  which  unite  to  make  a  successful  or  unsuccessful 
preparation.  The  various  osmic  and  chromic  fixatives,  particularly 
Flemming's  strong  mixture  and  Hermann's  fluid,  have  perhaps  been 
most  used,  and  with  the  greatest  success.  Corrosive  sublimate  and 
Zenker's  fluid  have  been  very  successful  in  many  cases.  Smear  prepara- 
tions have  been  used  in  a  few  cases  and  ought,  if  used  carefully,  to  be 
superior  as  a  means  of  merely  counting  well  separated  and  compact 
chromosomes.  They  cannot  be  trusted  to  show  the  relations  of  parts. 
The  earlier  stages  of  spermatogenesis  have  been  observed  during  life  by 
Wilson  in  the  case  of  Anasa. 

LITERATURE 

From  a  very  large  list  of  good  papers  we  will  select  only  a  few  in  which  more  complete 
bibliographies  may  be  found. 
WILSON,  E.  B.     "Studies  on  Chromosomes"  in  the  Journ.  of  Experimental  Zool.,  Part  I 

in  Vol.  II,  1905,  p.  371;  Part  II  in  Vol.  II,  1905,  p.  371;  Part  III  in  Vol.  Ill,  1906. 

The  sexual  differences  of   the  chromosome   groups  in  Hemiptera,  with  some  con- 
siderations of  the  determination  and  inheritance  of  sex. 
BRATJER,  A.     "Zur  Kentniss  der  Spermatogenese  von  Ascaris  megalocephala,"  Arch.  f. 

mik.    Anat.,  Band  XLII,   1893. 
MEVES,  F.    "  Uber  die  Entwicklung  der  mannlichen  Geschlechtszellen  von  Salamandra 

maculosa,"  Arch.  f.  mik.  Anat.,  Band  XLVIII,  1896. 
MOORE,  J.  E.  S.     "On  the  Structural  Changes  in  the  Reproductive  Cells  during  the 

Spermatogenesis  of  Elasmobranchs,"  Quart.  Journ.  Micr.  Sci.,  Vol.  XXXVII,  1895. 
PAULMIER,  F.  C.  "The  Spermatogenesis  of  Anasa  tristis,"  Journ.  of  M  or  ph.,  Vol.  XV, 

Supplement. 
McCLUNG,  C.  E.     "The  Accessory  Chromosome  Sex-Determinant,"  Biol.  Bull.,  Vol.  Ill, 

Nos.  i  and  2,  1902. 
STEVENS,  N.  M.     "Studies  in  Spermatogenesis  n."     A  comparative  study  of  the  hetero- 

chromosomes  in  certain  species  of  Coleoptera,  Hemiptera,  and  Lepidoptera  with 

special  reference  to  sex  determination.     Carnegie  Inst.  Wash.  Pub.,  Vol.  XXXVI, 

No.  2,  1906. 
CASTLE,  W.  E.     "  The  Heredity  of  Sex,"  Bull.  Mus.  Comp.  Zool.,  Harvard,  Vol.  XL,  No.  4, 

1903. 

SUTTON,  W.  S.     "The  Chromosomes  in  Heredity,"  Biol.  Bull.,  Vol.  IV,  No.  5,  1903. 
JORDAN,  H.  E.     "The  Spermatogenesis  of  Aplopus  Mayeri,"  Carnegie  Inst.  of  Wash., 

Pub.  No.  102,  1908. 


FEMALE  REPRODUCTIVE    CELLS 


453 


GROWTH   AND   MATURATION    OF   THE  FEMALE  REPRODUCTIVE 

CELLS 

The  female  reproductive  cells  are  usually  indistinguishable  from  the 
male,  so  far  as  any  visible  cytological  differentiation  is  concerned,  until 
near  the  time  of  yolk  accumulation  and  maturation.  They  can  usually 
be  determined  a  long  time  before  this  by  their  position,  or  by  the  devel- 
opment of  accessory  sexual  tissues  in  the  organism  that  are  not  in  direct 
contact  with  them. 


FIG.  416.  —  A  half-grown  ovum  of  Molgula  manhattensis.   f.c.,  outer  row  of  follicle  cells; 
n.c.,  nurse.cells  moving  from  follicle  into  cytoplasm  of  ovum.     X  1200. 

The  first  noticeable  feature  in  the  development  of  the  young  female 
reproductive  cell  is  its  relation  to  the  cells  which  are  going  to  aid  it  in 
securing  and  storing  in  its  body  the  great  amount  of  food  material  that 
will  be  needed  later.  It  appears  probable  that,  owing  to  its  necessary 
occupation  with  its  own  internal  preparations,  or  perhaps  owing  to  the 
enormous  quantity  of  food  required,  or  possibly  to  the  lack  of  food-pre- 
paring structures  in  its  own  make-up,  that  the  ovum  is  always  asso- 
ciated with  such  accessory  nurse  cells. 

A  primitive  method  of  food  acquisition  is  for  the  young  egg  cell  to 
wander  among  the  surrounding  tissue  cells  and  ingest  them.  This 


454 


HISTOLOGY 


appears  to  be  the  case  in  some  hydroids.  This  method  of  ingesting  food- 
laden  cells  is  carried  up  into  some  higher  forms  as  the  tunicates,  where 
the  growing  egg  cell  ingests  whole  rows  of  surrounding  yolk  cells  whose 
nuclei  persist  for  a  while  in  its  cytoplasm  (Fig.  416).  As  a  rule,  where 
the  cells  are  thus  eaten  bodily  they  are  neighboring  reproductive  cells 
which  are  thus  arrested  in  their  career.  This  appropriation  may  even 
go  on  after  the  eggs  are  laid  and  development  has  commenced. 

The  more  general  method  of  yolk  accumulation,  however,  is  for  the 
growing  ovum  to  join  itself  in  close  bodily  contact  with  some  cell  that, 


FIG.  417.  —  Five  stages  in  the  growth  development  of  an  oogonium  of  Myzostoma.  The  two 
nurse  cells  (w.c.)  increase  in  size  as  the  ovum  grows  and  finally  fuse  with  its  cytoplasm.  (After 
WHEELER.) 


if  related  by  descent,  comes  from  a  more  remote  common  ancestor  and 
is  differentiated  to  act  as  a  follicle  cell  or  nurse  cell.  In  this  case  the 
follicle  cell  or  nurse  cell  takes  food  from  the  blood  and,  after  elaborating 
it  in  some  unknown  manner,  feeds  it  into  the  egg  cell  until  there  is  a  suf- 
ficient supply.  It  usually  ends  by  giving  up  its  own  substance  until  but 
a  dead  remnant  appears  lying  on  the  surface  of  the  young  ovum. 

In  the  annelid  Ophryotrocha,  Korschelt  has  shown  that  one  single 
nurse  cell  does  all  the  work,  attaching  itself  to  the  very  small  ovum  and 
feeding  it  both  with  food  secured  from  the  surrounding  body  cavity 
fluid  and  with  the  contents  of  its  own  body  until  it  appears  as  a  mere 
excrescence  on  the  body  of  the  full-grown  ovum,  now  many  times  the 
bulk  of  the  two  together  when  they  started. 


FEMALE  REPRODUCTIVE    CELLS 


455 


FIG.  418.  —  Two  nurse  cells  of  the  single  layer  which  feeds  yolk  into 
the  growing  ovum  of  the  crayfish,  Cambarus.  c.t.,  part  of  the  con- 
nective-tissue follicle;  n.c.,  two  nurse  cells  of  the  ovarian  follicle; 
y.,  yolk  granules  in  the  ovum.  X  1300. 


In  Myzostoma,  Wheeler  has  described  the  ovum  as  growing  and 
securing  its  yolk  supply  through  the  agency  of  two  nurse  cells,  one  of 
which  is  attached 
to  either  end.  Fig- 
ure 417  shows  five 
stages  in  this  pro- 
cess and  it  can  be 
seen  that  the  nurse 
cells,  unlike  that  of 
Ophryotrocha,  grow 
in  size  as  the  ovum 
does  and  finally 
fuse  with  the  cyto- 
plasm of  the  full- 
grown  egg.  There  appears  to  be  a  slight  differentiation  in  the  yolk 
which  the  two  nurse  cells  produce,  this  differentiation  resulting  in  a 
polarity  of  the  ovum,  which  is  retained  through  its  further  development. 
In  by  far  the  largest  number  of  animals  the  yolk  is  prepared  and 
loaded  into  the  egg  by  a  single,  epithelial-like  layer  of  nurse  cells.  This 
layer  varies  in  thickness  from  the  very  thin  layer  found  in  most  echino- 
derms  through  many  grades  of  thickness  and  probably  increased  effi- 
ciency, to  the  long  columnar 
cells  that  make  up  the  nurse- 
cell  layer  of  a  siluroid  fish. 

Omitting  the  case  of  an 
echinoderm  because  it  is 
mentioned  later,  we  shall 
demonstrate  such  a  series 
beginning  with  the  yoke- 
forming  nurse  cells  of  a 
crustacean,  Cambarus,  part 
of  a  section  of  whose  half- 
grown  ovarian  egg  is  shown 
in  Figure  418.  The  nurse 
cells  form  a  flat,  thin  layer 
here  of  wide  cells,  with 
nuclei  that  are  not  much 
different  from  those  of  the 
surrounding  tissues. 

The  food  materials,  from 
the  blood  supply  which  is 


* Z.s. 


FIG.  419.  —  Part  of  the  single  layer  of  follicle  cells  which 
surround  the  growing  ovum  of  the  catfish,  Ameiurus 
nebulosus.  c.t.,  connective-tissue  follicle;  n.c.,  nurse 
cells  of  the  ovarian  follicle;  z.r.,  transparent  zona  ra- 
diata  or  cell  wall  of  ovum;  y.,  part  of  ovum,  showing 
two  large  yolk  spheres;  l.s.,  lymph  space  (artificial 
or  pathological?).  X  870. 


constantly  circulating  through  the  surrounding  space  indicated  by  l.s., 
must  go  through  two  layers  of  cells  to  get  into  the  egg,  a  connective- 


456  HISTOLOGY 

tissue  layer  lying  outside  of  the  follicle  layer.  It  is  probable  that  the 
entire  blood  soaks  through  spaces  in  the  connective-tissue  layer,  and 
that  only  then  do  the  nurse  cells  (follicle  cells)  select  and  elaborate  the 
proper  materials  and  pass  them  as  yolk  food  into  the  egg. 

The  same  thing  is  probably  true  of  a  very  thick,  single  layer  of  cells 
shown  by  the  follicle  layer  of  a  catfish,  Ameiurus  nebulosus  (Fig.  419). 
While  the  ovum  is  very  young,  these  cells  are  small  and  flat  as  in  the 
crayfish.  During  the  time  of  greatest  yolk  accumulation,  the  cells  grow  to 
the  great  height  shown  in  the  figure.  The  nuclei  become  smaller  and 
angular,  and  a  space  appears  between  the  follicle  cell  layer  and  the  con- 
nective-tissue capsule.  This  space  becomes  filled  with  a  heavy  lymph 
of  some  staining  power.  The  blood  supply  appears  as  a  network  of  cap- 
illaries in  the  connective-tissue  capsule  instead  of  in  sinuses  outside  of 
it  as  in  the  crayfish.  Figure  420  shows  part  of  a  section  of  the  ovarian 
egg  of  another  teleost  fish,  the  carp,  and  as  this  ovum  was  ripe  and  nearly 
ready  to  be  shed,  the  follicle  cells,  which  were  almost  as  long  as  those 
of  the  catfish  at  an  earlier  stage,  are  now  flat  and  shrunken  and  will  soon 
die  and  disintegrate. 

The  cells  of  the  single-layered  follicle  usually  multiply  by  mitosis  in 
the  early  part  of  their  career.  Later  such  cells,  in  some  forms,  divide 
by  amitosis,  which  is  a  terminal  process  under  the  circumstances,  as  the 
layer  is  destroyed  at  about  the  time  the  egg  is  laid.  (See  Amitosis, 
Chapter  V.) 

We  wish  to  call  attention  at  this  point  to  the  membrane  which  im- 
?  --   .-'.:•,..  mediately  surrounds  the  ovarian  egg  in 

most  animals  and  through  which  the 
food  materials  and  other  contents  of  the 
egg  must  be  passed.  In  the  catfish  (Fig. 
4*9)  this  membrane  is  comparatively 
thin,  and  one  might  think  that  the  ma- 
terials passed  inside,  in  a  fluid  state,  by 
osmosis.  In  the  adult  fish  egg  as  pic- 
tured in  the  carp  (Fig.  420),  this  mem- 
brane is  very  clearly  perforated  by  a 

FIG.  420.  —  Edge  of  ovum  of  another  tele-  J  *• 

ost  fish,  EsoxAmericanus,  to  show  the    vast  number  of  tiny,  radial  canals  and 

radial  canals  in  the  thick  cell  wall  of  the    cytoplasmic    prOCCSSCS  of    the    nurSC 
the  ovum.     This  cell  wall  is  two-lay-          »«"''« 

ered.     The  egg  being  ripe,  the  nurse  Cells    paSS     into     these     Canals    and    thus 

cells  are  small  and  degenerating,    c.t.,  carry    the    food    matter    in.       The    mem- 

connective-tissue  capsule ;  «.e.,degen-  ,  .  iiir  i    •    •      i      i         i 

erating  nurse-ceil  layer;  y.,  yolk  in    brane  is  probably  formed  jointly  by  the 

ovum;  z.r.,  two-layered  zona  radiata.      follicle    cells    and    the    OVUm,    the    latter 

being  responsible   for  only  the  thinner 

inner  layer.  In  some  fish  eggs  the  outer  surface  of  the  membrane  is 
drawn  out  into  long  threads  for  attaching  the  eggs  to  seaweed  and 


FEMALE  REPRODUCTIVE    CELLS 


457 


other  foreign  bodies.    These  are   described  by  Eigenmann  as  being 
formed  between  the  follicle  cells. 

A  definite  step  in  the  organization  of  such  a  layer  of  nourishing  cells 
is  met  with  in  the  insects,  where  the  egg  may  be  said  to  be  covered  by  a 
single  layer  of  nurse  cells,  one  or 
more  of  which  is  enlarged  to  per- 
form the  function  of  yolk  storage. 
Where  several  cells  out  of  the  layer 
assume  this  r6le,  they  may  con- 
tinue to  lie  in  a  single  layer  or 
they  may  become  arranged  in  a 
mass  that  is  practically  a  stratified 
epithelium.  A  good  subject  to 
study  is  a  "ground  hornet,"  Sco- 
lia  dubia.  In  this  insect  the  ova- 
rian tubules  are  terminated  dis- 
tally  by  chambers  of  primordial 
cells.  A  succession  of  single  ova, 
each  surrounded  by  a  layer  of 
follicle  cells,  arises  from  this  ter- 
minal chamber  and  moves  with 
the  entire  tubule  toward  the  egg 
duct  as  an  egg- follicle.  Alternating 
with  the  egg-follicles  are  masses 


FIG.  421.  —  Young  egg-follicle  and  attached 
nurse-cell  follicle  of  the  ground-hornet,  Scolia 
dubia.  ov.,  ovum  surrounded  by  a  single  layer 
of  passive  (?)  follicle  cells;  «./.,  follicle  com- 
posed of  young  active  nurse  cells  which  were 
derived  from  a  part  of  the  single  layer.  X  350. 


of  nurse  cells  surrounded  by  a 
follicle  epithelium  to  form  a  nurse- 
cell  follicle.  Each  nurse-cell  fol- 
licle is  attached  to  its  egg-follicle 
on  its  distal  end  (Fig.  421).  The  ovum  sends  a  cytoplasmic  process 
up  into  the  fundus  of  the  nurse-cell  follicle.  This  latter  follicle  has  in- 
creased in  size  by  the  growth  of  the  nurse  cells.  As  the  nurse  cells  grow, 
their  nuclei  become  irregular  in  contour  and  the  chromatin  breaks  up 
into  minute  granules  which  are  uniformly  distributed  throughout  the 
nucleus.  The  cells  which  are  first  to  so  differentiate  lie  next  the  ovum 
(Fig.  422).  This  differentiation  results  in  the  secretory  powers  of  the 
cells  becoming  active.  A  vacuole  now  appears  in  the  egg  cytoplasm,  as 
though  a  streaming  fluid  had  excavated  the  egg  in  this  region  (Fig.  422, 
vac.}.  The  growth  of  ovum  and  nurse-cell  follicle  continues.  Even- 
tually a  wave  of  cytoplasmic  disintegration  passes  with  some  regularity 
through  the  nurse-cell  follicle  from  the  ovum  distally.  This  is  followed 
by  the  disintegration  of  the  nuclei.- 

The  destruction  of  the  contents  of  the  nurse-cell  follicle  causes  it  to 
contract  and  finally  disappear  as  an  appendage  of  the  egg- follicle.     Fig- 


458 


HISTOLOGY 


n.  f. 


n.b 


ey.p 


ure  423  shows  the  destruction  of  the  contents  almost  completed.  In  the 
earwig,  Forficula,  but  one  follicle  cell  becomes  a  nurse  cell.  In  other 

insects,  as  Vanessa,  sev- 
eral follicle  cells  are  so 
modified,  but  remain  an 
integral  part  of  the  follicle 
and  are  not  separated 
from  it  as  in  Scolia. 

Multiple  layers  of  fol- 
licle cells  are  confined  to 
the  higher  and  most 
highly  specialized  animals 
and  are  instructive  in 
that  they  show  the  ne- 
cessity of  bodily  contact 
between  the  ovum  and 
the  yolk-supplying  cells. 
Figure  424  shows  the 
layer  of  yolk  cells  sur- 
rounding the  growing 
ovum  of  a  water  snake, 
Natrix  sipedon.  When 
the  ovum  first  begins 
to  grow,  this  layer  is 
single.  The  cells  increase 
in  size  and  form  a  strati- 
fied layer  by  amotitic  pro- 
liferation. This  layer  is 
thickest  just  before  the 
ovum  has  attained  its  full 
size,  and  when  this  size 
has  been  secured,  the  fol- 

cells.     cy.p.,  cytoplasmic  process  of  ovum  through  which      licle  Cells   degenerate   into 
the  nourishing  matter  is  drawn  as  a  streaming  vacuole      a     layer      of     dead      Cells 

which    form    a    mucous 

covering  to  permit  the  ovum  to  slide  out.  The  figure  shows  the 
most  actively  secreting  stage.  Study  the  middle  layer  and  note  that 
although  the  layer  is  stratified,  yet  every  cell  secreting  has  a  strand 
of  its  cytoplasm  drawn  out  into  a  process,  the  yolk  process,  which 
comes  into  contact  with  the  ovum  by  passing  through  the  egg  mem- 
brane. This  provides  a  necessary*  pathway  for  the  yolk  material, 
in  solution  or  in  fine  granules,  to  be  carried  into  the  egg  without  being 
passed  from  cell  to  cell.  Note  also  that  the  larger  cells  show  a  mass 


FIG.  422. —  Older  ovum  of  Scolia  dubia.  Lettering  same 
as  in  Figure  421.  At  n.b.  the  chromatin  has  not  yet  be- 
come distributed  as  it  is  in  the  very  actively  secreting 


FEMALE  REPRODUCTIVE    CELLS 


459 


of  food  material  being  elaborated  in  the  cytoplasm  distal  from  their 
nucleus. 

The  extreme  outer  layer  is  distinguished  by  its  lack  of  food  materials 
and  is  evidently  not  so 
actively  engaged  as  the 
middle  layer  in  which  the 
largest  cells  lie.  The  in- 
nermost layer  also  con- 
tains smaller  cells. 

These  cells  multiply  by 
mitosis  during  the  earlier 
development  of  the  ovum. 
A  mere  increase  in  the 
size  of  the  individual  cells 
appears  to  give  the  follicle 
sufficient  capacity  when 
the  maximum  growth  is 
attained.  In  the  mam- 
mals the  multiple  follicle 
layer  continues  to  increase 
the  number  of  its  cells  by 
mitosis  up  to  the  full 
maturity. 

The  ovum  of  a  mam- 
mal begins  its  growth 
period  with  a  single  layer 
of  nurse  cells.  As  the 
growth  proceeds,  these  in- 
crease by  mitotic  divisions 
to  a  double  or  many  lay- 
ered covering.  Later,  the 
outer  part  of  the  covering 
becomes  separated  from 
that  immediately  covering 
the  ovum,  except  at  a 
single  point  called  the  hilum.  The  space  between  the  two  layers 
becomes  filled  with  a  fluid  that  must  act  as  an  intermediate  carrier  for 
nearly  all  the  exchanged  materials. 

The  double  or  triple  layer  of  nurse  cells,  covering  the  egg  at  this  time, 
each  sends  a  cytoplasmic  process  down  to  the  egg  membrane.  This  egg 
membrane  is  moderately  thick,  but  seems  to  be  of  a  soft  consistency. 

No  radial  canals  are  shown  by  which  the  processes  might  pass 
in,  or  through  which  the  yolk  substance  might  pass  (Fig.  425). 


FIG.  423.  —  Part  of  nearly  matured  ovum  of  Scolia  dubia 
with  the  attendant  yolk  follicle  used  up  and  degenerat- 
ing. All  communication  between  ovum  (ov.)  and  yolk 
follicle  («./.)  is  cut  off.  X  350. 


460 


HISTOLOGY 


:*>•?.;  ^.  >>..•-..  ,f.v  .  .,.;.  ,:  •<.•.;:;:  .•>.?. 

&'i-£!':Q'Z\&"*  C:V--;:.:^^^;:^i*- 

i&ii^iij^^i^ji^.-'^)':.?"  *^ 


We  may  conclude  that  the  yolk  passes  in  as  a  fluid,  or  that  there 
are  canals  which  are  too  fine  to  be  detected. 

The  growing  ovum  often, 
but   not   always,   possesses 

,1  i  .   T     . 

another  structure  which  is 
but  little  understood,  but 
which  appears  to  be  con- 
cerned with  its  yolk  accum- 
ulation. This  is  a  body  in 
the  cytoplasm  which  is 
known  as  the  yolk  nucleus. 
Its  most  characteristic  ap- 
pearance, perhaps,  is  in 
some  of  the  fishes,  as,  for 
example,  in  the  yolk  nu- 
cleus of  Lophius,  the  angler, 
where  it  can  be  seen  in  the 
egg  a  third  grown,  as  a 
denser,  darker-staining  mass 
in  the  middle  cytoplasm 
(Fig.  426). 

It  has  been  described  in 
another  characteristic  form 
by  Calkins    in    the  earth- 
worm, Van  Bambeke  in  a 
In  this  last  type  it  first  appears 


FIG.  424.  —  Part  of  the  multiple  layer  of  nurse  cells  in 
the  egg-follicle  of  the  half-grown  ovum  of  a  water 
snake,  Natrix  sipedon.  One  of  the  four  large  secret- 
ing cells  (sec.c.)  shows  a  process  extending  through 
the  cell  wall  of  the  egg.  ov.,  edge  of  ovum;  conn.t., 
connective-tissue  layer  of  follicle;  sec.c.,  secreting 
nurse  cells.  X  720. 


spider,  and  Crampton  in  an  ascidian. 
as  an  entire  or  partial  ring  of  the  dark-staining 
material  encircling  the  nucleus.  This  ring  re- 
treats peripherally,  at  the  same  time  undergo- 
ing a  disintegration  into  smaller  bodies  or  even 
into  diffused  granules.  It  finally  becomes  dis- 
tributed throughout  the  cytoplasm  and  thus  is 
lost  as  a  visible,  structural  feature  of  the  cell. 
Figure  427  is  a  series  of  three  stages  as  figured 
by  Van  Bambeke  in  a  spider,  Pholcus. 

A  centrosome  has  been  described  as  occur- 
ring in  young,  resting  ova  as  well  as  those  seen 
during  divisions.  Munson  has  described  a  pe- 
culiar body,  in  the  ovum  of  Limulus,  which 

•r' 

has  the  appearance  of  either  a  centrosome  or  a 
nebenkern  or  a  yolk  nucleus. 

In  turning  once  more  to  the  important  structural  changes  which  take 
place  in  the  nucleus  of  the  growing  ovum,  we  find  ourselves  confronting 


FlG  4»s-—  Portion  of  the  nurse- 

cell  layer  of    the  egg-follicle 

Of  a  cat.   ov.,  edge  of  ovum. 
x  8?°- 


FEMALE  REPRODUCTIVE    CELLS 


46l 


the  most  interesting  and  difficult  subject  in  cytology.  Although  its 
essential  features  have  been  already  described  in  the  reduction  of  male 
reproductive  cells,  we 
shall  outline  it  again  as 
seen  in  the  female  cells, 
following  this  outline 
by  several  concrete  de- 
scriptions, and  finally 
by  some  account  of  the 
union  of  the  male  and 
female  gametes  to  form 
the  oosperm  or  zygote. 

The  young  female 
reproductive  cell,  before 
it  begins  its  growth,  is 
known  as  an  oogonium. 
It  possesses  the  same 
number  of  chromatic 
units  or  chromosomes  that  are  characteristic  of  the  somatic  cells  of  the 
species.  The  beginning  of  maturation  (which  is  a  comparatively  rapid 
process  occurring  during  the  breeding  season)  is  marked  by  a  gathering 
of  the  chromatin  into  a  closely  reticular  mass  that  lies  at  one  side  of  the 
nucleus.  This  process  was  formerly  known  as  synapsis  but,  as  synapsis 
is  now  used  as  a  term  to  designate  another  process  which  may  take  place 
during  this  closely  reticular  stage  or  before  it  (usually  during  the  telophase 
of  the  last  oogonial  division),  the  term  synizesis  has  been  used  to  desig- 
nate the  close  reticulum. 

Upon  emerging  from  synizesis,  the  oogonium  begins  a  quick  period 
of  growth  or  yolk  accumulation,  by  the  means  already  described,  and 


FIG.  426.  —  Young  ovum  of  the  angler,  Lophius  piscatorius. 
Shows  the  yolk  body  beneath  the  nucleus.     X  500. 


FIG.  427.  — Three  stages  in  the  growth  period  of  the  egg  of  Pholcus.  The  yolk  body  appears 
as  a  ring  about  the  nucleus,  and  swells  and  disintegrates  as  the  yolk  accumulates.  (From 
WILSON  after  VAN  BAMBEKE.) 

then  becomes  an  oocyte  of  the  first  order.  It  now  forms  its  chromosomes 
for  the  two  divisions  known  as  the  reduction  divisions.  If  the  somatic 
number  of  chromosomes  during  a  division  is  12,  we  shall  find  that  the 


462 


HISTOLOGY 


primary  oocyte  has  but  6  chromatin  masses,  which  are  larger,  however, 
than  usual,  and  in  some  animals  can  be  seen  to  consist  of  four  portions 

each.  In  this  latter 
case  they  are  called 
the  tetrads. 

A  mitotic  division 
now  takes  place  in 
which  the  tetrads  are 
split  in  two,  and  each 
daughter  cell  re- 

FIG.  428. — Astenas  rorbesii.     Region  of  young  ovary  from  .  ,     .       .  , 

which  the  reproductive  cells  originate.     X  1500.     (Drawn         CClVeS  0   dyads.       1  he 

by  H.  E.  JORDAN.)  two  resulting  cells  are 

known  as  oocytes  of  the  second  order,  and  they  at  once  proceed  to  perform 
another  mitotic  division  without,  meanwhile,  re-forming  the  nucleus. 
This  second  division  results  in  the  dyads  being  pulled  apart  and  each 
of  the  four  resulting  cells  getting  6  monads,  or  6  chromosomes,  as  we  must 
term  them.  This  is  one  half  the  somatic  number.  One  of  the  4  cells 
is  now  a  matured  ovum. 

These  equal  divisions  of  the  nuclear  elements  were  not  followed  by 
equal  divisions  of  the  cell  body  and  its  load  of  yolk.  When  the  primary 
oocyte  divided,  one  secondary  oocyte  took  practically  all  the  cytoplasm, 
leaving  its  sister  cell  to  appear  as  a  tiny  mass  of  nuclear  substance  which 
is  discharged  from  the  ovum.  This  smaller  secondary  oocyte  is  called 
the  first  polar  body.  In  the  second  reduction  division  the  same  thing  is 
repeated,  and  one  of  the  resulting  ova  is  discharged  (divided)  from  its 
sister  cell  as  the  tiny  second  polar  body,  while  the  first  polar  body  often 
makes  an  attempt  at  division  which  results  in  there  being  three  polar 
bodies  attached  to  an  ovum  instead  of  two. 

This  process,  especially  its  latter  part,  can  be  very  easily  traced  in 
the  growing  and  maturing  ova  of  a  starfish,  Asterias  Forbesii.  At  an 
early  age  the  reproductive  cells 
of  this  animal  are  situated  in 
the  epithelium  lining  a  compound 
tubular  organ,  which  is  the  ovary. 
They  are  exceedingly  small  and 
can  only  be  detected  among  the 
somatic  cells  when  they  begin  to 
grow  in  size  for  the  maturation 
process.  Figure  428  shows  them 
just  before  this  occurs.  It  occurs 
principally  during  a  few  spring 
and  summer  months  in  Asterias  Forbesii,  but  goes  on  to  some  ex- 
tent during  the  whole  year.  At  this  time  the  ova  begin  to  enlarge  and 


FIG.  429.  —  Asterias  Forbesii.  Group  of  young 
oogonia  (ogn.).  Three  of  the  oogonia  are  un- 
dergoing synizesis  (s yn.~).  X  1500.  (Drawn 
by  H.  E.  JORDAN.) 


FEAIALE  REPRODUCTIVE    CELLS 


463 


appear  to  go  through  the  various  processes  in  groups  which  may  be  lik- 
ened to  the  sperm  columns  of  the  testis.  The  cells  are  exceedingly  small 
during  the  early  stages,  so  small  that  they  can  hardly  be  seen.  They 
are  first  recognizable  as  reproductive  cells  by  the  large  black  nucleolus, 
and  shortly  after  the  outline  of  the  expanding  nucleus  can  be  seen.  When 
they  are  between  two  and  three  microns  in  diameter,  a  cytoplasmic  body 
becomes  apparent;  the  chromatin  appears  as  a  delicate  spireme,  and 
they  are  ready  to  begin  maturation  (Fig.  429,  og.}.  No  mitosis  is  ob- 
servable which  can  be  interpreted  as  an  oogonial  mitosis. 

The  cells  enlarge,  and  the  chromatin  becomes  a  delicate  thread  (Fig. 
429,  ogn.).  It  afterward  grows  stouter  and  gathers  closely  around  the 
nucleolus,  which  is  of  some  size  by  this  time.  This  is  the  beginning  of 
synizesis  or  the  contraction  stage,  and  presently  the  nucleus  is  completely 


,  430.  — Asterias  Forbesii.  Stages  of  the  primary  oocyte  at  early  growth  period.  The  nucleus 
grows  and  yolk  is  accumulated.  The  chromatin  thread  breaks  into  chromosomes,  which 
become  reduced  in  size.  X  1500.  (Drawn  by  H.  E.  JORDAN.) 

hidden  by  the  crowding  together  of  the  chromatin  masses  (Fig.  429, 
Syn.\ 

This  stage  does  not  last  long,  as  is  evidenced  by  the  rarity  with  which 
it  occurs  in  any  given  section,  notwithstanding  the  great  number  of  ova 
which  are  formed.  More  common  are  the  stages  which  show  the  skein 
of  chromatin  relaxed.  The  whole  cell  is  very  rapidly  growing  at  this 
time,  and  its  size  corresponds  fairly  well  with  the  changes  that,  take  place 
in  the  chromatin.  The  skein  or  spireme  enlarges  with  the  widening 
nucleus  and  becomes  thicker.  It  is  double,  and  its  strands  are  granular. 
Figure  430,  A,  shows  this  stage  just  as  the  spireme  has  begun  to  divide 
into  a  number  of  pieces.  The  many  small  pieces  shown  in  the  figure 
are  largely  the  result  of  an  artificial  cutting  and  breaking  of  the  chromatin 
in  the  process  of  sectioning. 

In  Figure  430,  B,  we  see  the  portions  of  the  chromatin  spireme  thick- 
ened and  shortened.  They  present,  also,  a  mossy  appearance  at  this 


464 


HISTOLOGY 


FIG.  431.  —  Asterias  Forbesii.  Fully 
grown  ovum  (primary  oocyte),  show- 
ing large  chromatic  nucleolus  and 
smaller,  dark  staining  group  of  chro- 
mosomes. Lower  magnification  than 
preceding  figure.  X  500.  (Drawn 
by  H.  E.  JORDAN.) 


time.  This  stage  appears  very  regu- 
larly in  the  course  of  development  of 
all  the  ova.  The  chromatin  portions 
begin  at  this  time  to  decrease  in  size, 
and  soon  they  come  to  lie  in  one,  two, 
or  even  three  small  groups  near  the 
periphery  of  the  nucleus  as  in  Figure 
430,  C.  They  will  now  be  called  chro- 
mosomes, because  they  are  supposed  to 
be  the  individual  chromosomes,  small 
and  condensed  in  form,  wrhich  afterward 
take  part  in  the  maturation  divisions. 
When  seen  to  the  best  advantage,  they 
appear  as  tiny,  bi-lobed  bodies  lying  side 
by  side. 

The  ovum,  shortly  after  this,  attains 
its  full  size,  as  seen  in  Figure  431,  and 
the  chromosomes  now  appear  as  a  very  small  mass  indeed,  and  are 
easily  overlooked  in  the  large  nucleus,  especially  in  unfavorably  stained 
specimens.  They  seem  to  lie  in  almost  any  part  of  the  nucleus,  al- 
though they  are  oftenest  

next  to  some  part  of  the  '~~~^\    ^N  \ ' ,  ' ' 

nuclear   membrane.     In  w   ^' >N  ^i',//-' ^  ' 

order  to  determine  their 
presence  and  number, 
one  must  study  a  com- 
plete series  of  sections  of 
any  particular  egg  nu- 
cleus. Sometimes  they 
are  attached  to  the  nu- 
cleolus and  sometimes 
are  farthest  from  it.  As 
has  been  said,  they  may 
form  one  or  more  groups. 
The  figure  shows  them  in 
a  single  group. 

The  egg  is  now  ready 
for  the  maturation  divi- 
sions. The  nucleus  has 
become  situated  near  the 
periphery  of  the  ovum, 
and  the  ovum  is  ready 
at  any  time,  upon  a  mus- 


~"    '     '"'^v  /  «r  \ 

-  /  /,•?•••   y  *V  x 

^m><^* 


•  /  /,•,••':!  "\  •     *  .••ivu  >. 

kmstti  ^teaix 

.»•   .'•.  _.;.i  •••,'•  v. 


•••::•  vv.;.;.vVv:V;--<V-.rv^ 

^i^P 

FIG.  432.  —  Asterias  Forbesii.  Portion  of  an  ovum  just  be- 
ginning the  first  reduction  division.  Chromatin  leaving 
the  plastin  ground-substance  of  the  nucleolus  and  being 
added  to  the  chromosomes  near  the  forming  spindle. 
X  1500.  (Drawn  by  H.  E.  JORDAN.) 


FEMALE  REPRODUCTIVE   CELLS  465 

cular  pressure  or  the  crowding  of  the  ova  behind  it,  to  be  passed  out 
of  the  animal's  body  into  the  surrounding  water,  where  there  is  a  large 
probability  of  its  being  near  the  spermatozoa  which  have  been  similarly 
deposited  by  neighboring  males. 

The  sea  water  influences  the  ovum  to  go  through  the  maturation 
divisions.  The  nucleolus  and  the  chromosome  groups  are  usually  to 
be  found  at  this  time  nearest  the  distal  edge  of  the  eccentric  nucleus. 
In  the  most  ordinary  cases  the  nucleolus  begins  to  show  an  irregular 
outline,  the  distal  part  of  the  nuclear  membrane  begins  to  undulate 
and  shrivel,  and  a  radiating  figure  of  achromatic  material,  the  aster, 
appears  in  the  cytoplasm  just  distal  of  the  nucleus  (Fig.  432).  In  fact, 
it  is  the  astral  rays,  combined  probably  with  some  solvent  agency, 
which  seem  to  cause  the  nuclear  membrane  to  give  before  them.  Pres- 
ently it  can  be  seen  that  the  many  chromosomes  have  separated  some- 
what and  moved  up  to  the  aster.  This  latter  has  divided  and  sepa- 
rated, leaving  a  set  of  connecting  fibrils,  the  spindle  fibrils,  which  extend 
between  the  daughter  asters. 

As  the  chromosomes  move  up  into  the  area  between  the  separated 
asters,  the  nucleolus  begins  to  discharge  its  chromatin  as  a  series  of 
irregular,  semi-fluid  lumps  which  leave  behind  them  the  plastin  body 
that  held  them  during  the  growth  period  (see  Fig.  432).  This  plastin 
remnant  does  not  stain  deeply,  and  after  becoming  irregular  it  breaks 
up  and  disappears.  Meanwhile,  the  chromatin  which  left  it  is  separated 
into  smaller  granules,  and  part  of  these  appear  to  act  as  a  source  of  nutri- 
ment to  the  chromosomes,  which  increase  considerably  in  size  at  this 
time.  Part  of  this  chromatin  is  distributed  through  the  rest  of  the 
nucleus,  which  has  now  lost  its  membrane  and  appears  as  a  larger,  more 
granular  and  darker-staining  area,  the  "  residual  substance,"  in  the 
cytoplasm. 

Often  the  passing  of  chromatic  material  from  nucleolus  to  chromo- 
somes begins  to  take  place  before  the  other  maturation  phenomena 
have  begun.  This  is  most  apt  to  be  seen  in  cases  where  the  chromo- 
some masses  and  nucleolus  are  at  some  distance  from  the  point  at  which 
the  spindle  is  formed.  Figure  433  shows  such  a  case  and  also  shows 
how  the  chromatin  passes  out  of  the  nucleolus  in  a  fine,  granular  stream. 
Part  of  the  plastin  ground  substance  has  been  left  free  by  the  chromatin 
in  this  instance. 

Shortly  after  such  a  figure  as  432  the  spindle  is  formed,  and  the  chro- 
mosomes, at  first  widely  scattered,  begin  to  be  drawn  into  a  fairly  regular 
equatorial  plate  (Fig.  434,  A).  They  have  already  begun  to  divide  by 
a  longitudinal  division  before  the  plate  is  actually  formed.  Shortly  after 
this  the  figure  appears  as  in  Figure  434,  B,  where  some  of  the  chromo- 
somes have  already  divided,  and  the  others  are  soon  to  follow.  They 


466 


HISTOLOGY 


are  bi-lobed,  and  split  longitudinally  into  two  equal  halves.  The  split 
appears  first  at  one  end,  and  the  divided  ends  are  opened  into  a  V-- 
shaped figure,  and  then 
pulled  apart  until  the  V 
is  straightened  out  into 
a  line.  At  this  point 
they  appear  as  a  rod 
with  three  lumps  on  it, 
one  on  each  end  and  a 
larger  one  in  the  middle. 
The  break  now  comes 
in  the  middle  of  the 
central  knob,  and  the 
bi-lobed  daughter  chro- 
mosomes move  apart 
toward  their  respective 
poles  (Fig.  434,  C). 

In   Figure    434,   D, 
may  be  seen  a  late  telo- 

FIG.  433.  —  Asterias  Forbesn.    A  slightly  younger  ovum  than        ,  .     ,.         .    .  . 

the  preceding  to  show  the  streaming  out  of  chromatin  to  a     pnase    OI    tniS    division, 
distant  group  of  chromosomes.    X  1500.    (Drawn  by  H.  E.      ancj   here   it   will   be   no- 

ticed  that  the  distal  end 

of  the  figure  has  emerged  from  the  surface  of  the  ovum.,  carrying  a 
little  cytoplasm  with  it.    This  is  the  first  polar  body. 


FIG.  434.  — Aslerias  Forbesii.     Four  stages  in  the  first  reduction  division.     X  1500. 
(Drawn  by  H.  E.  Jordan.) 

Almost  before  the  first  polar  body  is  completely  separated,  the  chro- 
mosomes in  the  ovum  begin  to  form  a  new  equatorial  plate  on  a  new 
spindle  derived  from  the  remaining  portion  of  the  first  spindle.  Figure 


FEMALE  REPRODUCTIVE   CELLS  467 

435,  A,  shows  such  a  new  spindle  with  the  chromosomes  at  metaphase. 
The  bi-lobed  chromosomes  are  again  divided  longitudinally  into  daughter 
chromosomes,  which  are  also  bi-lobed.  Figure  435,  B,  shows  a  telophase 
of  such  a  figure  which  soon  throws  off  a  second  polar  body  exactly  as  the 
first  figure  did  its  first  polar  body  (Fig.  435,  C).  In  this  case  the  first 
polar  body  is  apparently  making  an  abortive  attempt  to  divide.  Shortly 
after  this  the  egg  chromosomes  expand  into  vesicles  which  fuse  to  form 
the  female  pronucleus. 

Figure  435,  D,  shows  the  two  polar  bodies  completed  and  the  egg 
chromatin  forming  a  nuclear  membrane.  In  this  case  the  first  polar 
body  has  made  no  attempt  to  divide.  The  egg  aster  still  persists,  and 
the  whole  figure  appears  as  an  almost  perfect  resting  nucleus  with  a 
centrosome. 

A  remarkable  point  in  the  whole  growth  period  of  these  eggs  is  the 
lack  of  well-developed  nurse  cells.  The  follicle  cells  show  only  a  very 


FIG.  435.  —  Asterias  Forbesii.     Four  stages  to  show  the  second  reduction  division  and  the 
formation  of  the  female  pronucleus  (D).     (Drawn  by  H.  E.  JORDAN.) 

thin  layer,  complete,  but  so  delicate  that  they  do  not  seem  to  have  the 
power  of  preparing  the  great  quantity  of  yolk  that  the  ova  acquire.  A 
possible  explanation  of  this  condition  is  the  fact  that  the  ovaries  lie 
bathed  in  the  general  blood  supply,  and  that  this  has  such  free  access 
to  the  ova  that  they  can  absorb  sufficient  food  material  directly  from  its 
body.  This  presupposes  that  the  blood  is  very  rich  in  a  food  supply 
which  needs  but  little  elaboration  to  become  yolk. 

When  the  egg  has  matured,  and  is  brought  into  the  proximity  of 
sperm,  the  spermatozoa  are  influenced  by  its  presence  to  direct  them- 
selves toward  it,  and  to  make  energetic  swimming  efforts  to  reach  it. 
Upon  reaching  it,  the  first  spermatozoon  forces  its  way  through  the  thick 
zona  radiata  at  the  micropyle,  a  small  opening  which  is  there  for  that 
purpose.  As  it  draws  near  the  ovum  the  latter  responds  to  its  first  con- 
tact by  a  lifting  of  the  cytoplasm  at  that  point  in  the  form  of  a  cone,  into 
which  the  sperm  passes.  It  continues  to  move  into  the  cytoplasm  of 
the  ovum,  but  leaves  its  tail  behind  at  the  surface.  This  surface  now 
becomes  covered  with  a  delicate  membrane,  the  mtelline  membrane, 


468 


HISTOLOGY 


which  probably  prevents  any  more  spermatozoa  from  entering.  Figure 
436  shows  a  spermatozoon  in  the  ovum  and  another  vainly  attempting 
to  enter.  Sometimes  two  or  more  do  get  in  with  abnormal  results. 


FIG.  436. — Asterias  Forbesii.  One  sper- 
matozoon entered  into  ovum;  second 
vainly  attempting  an  entrance.  X  1500. 
(Drawn  by  H.  E.  JORDAN.) 


FIG  .437-  —  A  sterias  Forbesii.  Sperm 
head,  now  becoming  male  pronu- 
cleus,  approaching  the  female  pro- 
nucleus.  X  440.  (Drawn  by  H.  E. 
JORDAN.) 


The  use  of  some  chemicals  results  in  the  postponing  of  the  membrane 
formation,  and  this  almost  always  leads  to  such  a  poly spermic  fertiliza- 
tion. 

As  the  sperm  head,  with  its  middle  piece  attached  behind,  advances 
toward  the  egg  nucleus,  it  rotates  so  that  the  middle  piece  is  in  front. 
The  cytoplasm  through  which  it  has  passed,  spreads  out  behind  it  in  a 
widening  "wake"  of  somewhat  different  staining  power  from  its  original 

condition.  This  is  called  the  "en- 
trance funnel."  The  sperm  head  now 
begins  to  enlarge,  and  a  centrosome, 
arising  from  intimate  connection  with 
the  middle  piece,  acquires  rays  which 
originate  in  and  grow  out  into  the  cyto- 
plasm (Fig.  437).  These  rays  consti- 
tute the  future  dynamic  apparatus  for 
cell  division.  By  the  time  the  sperm 
head  has  arrived  next  to  the  egg  nu- 
cleus, it  has  become  enlarged  and  has 
opened  out  its  chromatin  pattern  to 
form  a  sperm  nucleus  which  is  but 
little  smaller  than  the  egg  nucleus  (Fig. 
438).  Asterias  thus  differs  from  some 
other  echinoderms,  as  Toxopneustes,  in  which  the  sperm  head  does  not 
open  up  much  until  it  has  joined  the  egg  nucleus.  The  process  of  union  is 


FIG.  438. — Asterias  Forbesii.  Apposition 
of  the  male  and  female  pronuclei.  X  44°. 
(Drawn  by  H.  E.  JORDAN.) 


FEMALE  REPRODUCTIVE   CELLS 


469 


one  of  close  apposition,  and  the  chromosomes  of  each,  18  in  number, 
remain  independent  of  each  other  for  a  long  time  after,  forming  the 
36  chromosomes  of  the  regular  starfish  cells.  They  each  divide  in  sub- 
sequent divisions,  so  that  every  descendant  has  36  chromosomes,  and 
each  of  these  36  was  derived  from  one,  and  a  different  one,  of  the  ori- 
ginal 36  in  the  newly  fertilized  ovum  or  ob'sperm.  Later,  when  some 
of  the  descendants  have  become  the  male  or  female  reproductive  cells 
of  the  young  animal,  and  they  are  beginning  to  mature,  they  will  at  last 
unite  the  paternal  and  maternal  chromosomes  into  18  bivalent  chromo- 
somes, which  are  the  tetrads  of  maturation.  This  process  is  called 
synapsis,  and  sometimes  occurs  during  synizesis  or  contraction.  It 
has  been  seen  in  a  few  cases,  in  some  Hemiptera,  where  it  is  described 
by  Wilson,  McClung,  and  others. 

A  study  of  the  mammal  ovum  will  yield  a  fuller  account  of  its  very 
early  development  than  that  of  the  starfish  does,  and  therefore  will 
serve  as  a  concrete  example  of  those  first  stages.  Like  the  male  re- 
productive cells  of  the  skate, 
the  female  cells  of  the  mammal 
may  first  be  seen  in  the  outer 
part  of  the  germinal  ridges,  two 
longitudinal  thickenings  of  the 
embryonic  body-cavity  wall  of 
the  body  cavity.  Later,  these 
folds  become  developed  into  the 
ovaries,  two  separate  bodies  of 
mesodermal  cells,  each  covered 
with  a  mesothelium.  The  re- 
productive cells  appear  as  pri- 
mordial egg  cells,  in  or  just  un- 
der the  mesothelium,  which  is  a 
part  of  the  peritoneum  reflected 
over  the  ovary. 

At  a  very  early  date  (before  birth  in  many  mammals),  all  the  repro- 
ductive cells  which  are  going  to  pursue  a  further  development  are  drawn 
down  in  groups  into  the  body  of  the  ovary.  These  groups  we  shall  term 
egg  tubules  in  general.  Figure  439,  A,  shows  such  a  group,  or  tubule, 
just  below  the  germinal  epithelium  in  the  ovary  of  an  embryo  of  the  cat, 
in  which  we  can  most  easily  trace  the  early  stages  in  the  history  of  the  mam- 
mal ovum.  These  cells  are  young  oogonia,  and  they  move  inward  from 
the  epithelium,  developing  as  they  pass  toward  the  inner  part  of  the  ovary. 
This  movement  is  continued  until  there  is  a  thick,  cortical  layer  of  young 
ova  a  short  distance  under  the  epithelium  and  extending  about  a  third 
of  the  diameter  of  the  ovary  inward. 


B 


FIG.  439.  —  Two  young  egg  tubules  near  the  sur- 
face of  the  ovary  in  a  kitten,  g.e.,  germinal  epi- 
thelium. A,  youngest  egg  tubule.  B,  older 
tubule.  X  1000. 


470 


HISTOLOGY 


FIG.  440.  —  Oogonia  from  ovary  of 
kitten.  Multiplying  by  mitotic 
divisions.  X  1000. 


When  first  seen  in  this  embryonic  material  the  reproductive  cells 
are  oogonia  of  exceedingly  small  size,  smaller  than  the  epithelial  cells, 
and  they  appear  to  originate  from  the  epi- 
thelium or  else  directly  under  it,  probably 
the  former.  They  form  the  groups  which 
we  have  called  the  tubules,  even  before 
they  have  left  the  surface.  In  other  mam- 
mals they  have  been  described  as  carrying 
an  invaginated  tube  of  the  epithelium  into 
the  ovary  with  them.  They  certainly  do 
not  do  this  in  the  cat,  and  when  they  have 
passed  in,  clear  of  the  basement  mem- 
brane, it  can  be  seen  that  they  are  sur- 
rounded by  connective  tissue.  These  ova- 
rian tubules  have  been  compared  to  the 
seminiferous  tubules  of  the  male. 

The  nucleus  of  each  cell  is  very  round,  and  the  nucleolus  lies  near  the 
middle,  with  a  reticulum  of  strands  of  linin  and  chromatin  reaching  out 
from  it  to  the  nuclear  membrane.  Cells  nearest  the  center  of  the  ovary 
in  each  group  are  somewhat  the  largest,  and  the  groups  inside  of  the 
first  are  larger  than  these  outer  ones.  The  figure  (439,  B)  shows  such  a 
larger  group  which  has  moved  into  the  ovary  a  trifle  farther  inside  than 
the  first. 

These  cells  undergo  a  few  oogonial  divisions  and  rapidly  increase  in 
size  as  they  move  inward  (Fig.  440).  The  nucleus  increases  at  a  greater 
proportional  rate,  and  opens  up  its  chromatin  pattern.  A  denser  area 
of  the  cytoplasm  then  ap- 
pears in  one  side  of  the  cell, 
and  the  nucleus  contracts 
its  chromatin  into  the  syn- 
izesis  stage.  Figure  441 
shows  a  part  of  the  cells 
from  a  tubule  whose  cells 
are  either  in  synizesis  or 
preparing  to  perform  it.  It 
can  be  seen  that  a  black 
central  granule  is  found  in 
the  dense  area  of  cytoplasm, 
and,  in  consequence,  the 
structure  has  often  been  de- 
scribed as  a  centrosome.  By 
other  writers  it  is  called  a  yolk  nucleus,  and  considered  homologous 
with  the  yolk  nucleus  of  spiders,  fishes,  and  other  mammals.  It  per- 


FlG.  441.  —  Oogonia  of  kitten  undergoing  synizesis. 
Yolk  body  present  in  some.     X  1000. 


FEMALE  REPRODUCTIVE    CELLS 


471 


sists  from  the  beginning  to  some  time  after  synizesis.  Some  of  the 
ob'gonia  are  now  seen  to  have  not  grown  or  otherwise  changed.  These 
are  destined  to  become  the  nurse  or  follicle  cells. 

We  shall  not  discuss  in  this  form  the  debatable  questions  connected 
with  the  details  of  chromatin  changes  which  take  place  before,  during, 
and  after  synizesis.  The  figure  shows  that  hi  the  preceding  stages  the 
thread-like  arrangement  of  the  chromatin  becomes  double,  and  its  loops 
appear  to  be  connected  individually  with  the  centrosome-like  body. 
Later,  as  is  shown  by  other  cells  in  the  figure,  the  thread  breaks  up  into 
shorter,  rod-like  bits  which  are  still  double.  The  height  of  the  contrac- 
tion stage  is  well  shown  by  two  of  the  other  cells  in  Figure  441.  All 
these  closely  successive  stages  form  a  layer,  just  inside  of  the  young 


FIG.  442.  —  Pre-oocytes  of  cat.     X  1000.     (Compare  with  Fig.  i,  which  is  the  fully  grown, 
first  oocyte  ready  for  the  reduction  divisions.) 

oogonia,  and  farther  still  inside  we  may  see  other  stages  which  show 
what  happens  when  synizesis  is  past. 

These  later  stages  show  several  histological  differences.  The  tubules 
are  being  divided  into  smaller  groups  by  the  growth  of  connective-tissue 
septa,  and  it  can  here  be  seen  that  some  of  the  original  tubule  cells  which 
did  not  undergo  synizesis  have  begun  to  divide  and  surround  the  young 
ovum,  which  must  now  be  called  a  pre-oocyte.  This  name  is  necessary 
to  distinguish  it  both  from  the  oogonium  before  synizesis,  and  especially 
from  the  later  primary  oocyte  which  goes  through  the  first  maturation 
division. 

The  pre-oocytes  have  all  been  formed  at  about  the  time  of  birth  in 
the  cat,  and  constitute  a  layer  just  under  the  surface  of  the  ovary.  This 
layer  remains  here  during  the  greater  part  of  the  cat's  life,  the  majority 
of  its  cells  never  changing,  wrhile  from  time  to  time  some  of  them  move 
a  little  farther  down  into  the  stroma  of  the  ovary  and  begin  a  rapid  growth 
period  which  ends  in  maturation  and  discharge.  Figure  442  shows 
several  cells  from  this  layer  in  an  adult  cat.  Among  the  pre-oocytes 


4/2 


HISTOLOGY 


ration    division    in   a  mouse    ovum. 
(After  SOBOTTA.) 


are  still  to  be  found  the  unchanged  oogonia,  now  become  nurse  cells. 

The  pre-oocytes  occur  either  singly  or  in  groups  of  two  and  three  or  more. 

When  another  ovum  is  to  be  matured, 
one  of  the  single  cells  or  one  of  the 
groups  moves  down  and  becomes  a 
primitive  follicle.  If  more  than  one  pre- 
oocyte  is  in  this  primitive  follicle,  one 
of  them  develops  at  the  expense  of  the 
others,  and  it  is  rare  to  find  a  follicle 
containing  more  than  one  ovum  at  ma- 
turity. In  some  other  mammals  very 
different  histological  conditions  obtain 
during  this  history,  especially  in  such 
as  have  no  great  mass  of  connective 

FIG.  443.  —  Prophase  of  the  first  matu-     tissue  in  the  Ovary. 

The  development  of  the  follicle  has 
already  been  spoken  of.  The  nurse 
cells  form  a  single  layer  around  the  ovum  and  then  stratify  to  form 
a  double  layer,  which  continues  to  multiply  its  cells  by  mitosis  until 
there  is  a  thick  multicellular  layer. 

Finally,  the  layer  which  is  directly  around  the  ovum  splits  concen- 
trically from  the  layer  which  lines  the  connective-tissue  capsule  of  the 
follicle,  and  the  two  become  separate  except  at  one  point,  called  the  hilum. 
When  the  ovum  is  ripe,  the  capsule  and  the  outer  wall  of  the  ovary 
break,  and  the  egg  is  discharged  into  the  body  cavity,  whence  it  passes 
into  the  oviduct  and  down  to  the 
uterus. 

From  the  time  that  it  leaves 
the  layer  of  pre-oocytes,  the  re- 
productive cell  undergoes  a  rapid 
growth,  and  at  or  about  the  time 
it  is  discharged  from  the  mature 
follicle,  it  undergoes  the  two 
maturation  divisions.  The  cat 
does  not  easily  exhibit  this  part 
of  the  process,  and  so  we  shall 
turn  to  rodent  material  to  exam- 
ine these  in  the  mammal. 

The  first  maturation  spindle 
in  the  maturing  ovum  of  the 
mouse  is  composed  of  distinct 

fibers  without  astral  rays.     Despite  Sobotta's  claim  that  the  spindle 
fibers  do  not  converge  but  tend  to  lie  parallel,  as  straight  lines  from  pole 


FIG.  444.  —  Anaphase  of  first  maturation  division 
in  the  muskrat,  Fiber  zibethecus.     X  750. 


FEMALE  REPRODUCTIVE    CELLS 


473 


to  pole,  the  fibers  do,  according  to  Lams  and  Doorme,  and  Kirkham, 
converge  to  points  at  which  lie  centrosome-like  granules. 

The  first  maturation  spindle  bears 
about  12  reduction  chromosomes.  The 
number  of  chromosomes  is  yet  a  matter 
of  dispute.  Lams  and  Doorme  have 
counted  in  two  cases  12,  and  state  the 
number  as  varying  from  12  to  15.  Like- 
wise Kirkham  is  inclined  to  make  his 
count  12.  Sobotta  gave  the  number  as 
12. 

The  spindle  at  this  stage  usually  lies 
parallel  to  the  surface  of  the  ovum  (Fig. 
443);  after  metaphase  the  chromosomes 
lie  at  the  two  ends  of  the  spindle,  which 
has  rotated  so  as  to  be  at  right  angles  to 

,  r  ,-,.  i  ,.  FIG.  445.  —  Ovum  of  mouse,  show- 

the  surface.      Figure  444  shows  this  stage       ing  first  ^^  bodV)  sec0nd  polar 


spindle,  and   entering   spermato- 
goon.    (After  LAMS  and  DOORME.) 


in  the  maturation  of  the  muskrat  ovum. 
The  spindle  fibers  at  this  stage  both  in 
the  muskrat  and  mouse  tend  to  be  parallel  straight  filaments. 

The  first  polar  body  is  formed  as  a  bud  from  the  surface  of  ovum 
into  which  the  distal  end  of  the  first  maturation  spindle  travels.  In  this 
position  the  spindle  is  divided,  at  the  cell  plate,  by  the  constricting  of 
the  bud  to  form  the  first  polar  body. 

The  remains  of  the  first  maturation  spindle,  left  within  the  ovum, 
re-forms  as  a  pointed  spindle,  which  according  to  Lams  and  Doorme 
becomes  about  the  size  of  the  first  spindle.  At  its  equator  twelve  or 
more  chromatin  bodies  assemble.  As  the  second  maturation  spindle 

retreats,  it  also  assumes  a  tangential 
position  (Fig.  445). 

The  metaphase  and  anaphase 
stages  of  the  second  reduction  mitosis 
ensue  during  the  time  that  this  sec- 
ond spindle  rotates  at  right  angles  to 
the  surface  of  the  ovum.  In  a  like 
manner  a  second  cytoplasmic  bud  is 
formed,  into  which  the  distal  half  of 
the  second  maturation  spindle  passes 

FIG.  446. — Mature  ovum  of  mouse,  show-      .      •  •         t       a-       '^.-L.    ±i_ 

ing  first  and  second  polar  bodies,  female      to  be  Constricted    Off   With   the   SCpara- 
pronucleus  (/./».),  and  male  pronucleus      tion  of  the   SCCOnd   polar   body. 

While  the  second  maturation  mito- 

sis  is  taking  place,  an  entire  sperm,  according  to  Lams  and  Doorme, 
and  Kirkham,  enters  the  cytoplasm  of  the  ovum. 


474  HISTOLOGY 

With  the  complete  formation  of  the  second  polar  body  the  sperm 
head  enlarges  and  becomes  reticular  to  form  the  male  pronucleus  (Fig. 
446,  m.p.}.  At  the  same  time  the  chromatin,  and  the  part  of  the  second 
maturation  spindle  which  remains  within  the  ovum,  become  organized 
into  the  female  pronucleus  (Fig.  446,  f.p.}.  The  blending  of  these  two 
pronuclei  results  in  the  nucleus  of  the  fertilized  ovum. 

Among  many  animal  forms,  most  of  them  highly  specialized,  the 
female  gives  birth  to  some  of  her  young  or  lays  eggs  that  hatch  and  de- 
velop without  the  presence  or  aid  of  any  male.  This  condition  is  com- 
mon among  certain  insects  and  crustaceans  as  well  as  some  other  ani- 
mals, and  is  known  as  parthenogenesis. 

The  young  so  produced  are  not  the  only  offspring  of  their  parent,  for 
at  other  times  certain  young  are  derived  from  eggs  that  have  been  fer- 
tilized by  the  male  cell  or  spermatozoon,  and  which  develop  as  usual. 

Sometimes  all  the  parthenogenetic  young  are  females,  a  state  which 
is  termed  thelytoky.  In  other  cases  only  males  result  from  the  partheno- 
genetic process,  and  this  condition  is  termed  arrenotoky.  When  both 
males  and  females  are  produced,  the  process  is  known  as  amphotoky. 

The  cytological  question  which  at  once  suggests  itself  is,  do  such 
offspring  come  from  eggs,  and  if  so,  do  the  eggs  mature  and  develop  with 
half  the  number  of  chromosomes  or  do  they  secure  a  full  complement  of 
chromosomes  in  some  other  way? 

Investigation  shows  that  the  eggs  do  mature ;  that  sometimes  they 
give  off  two  polar  bodies  and  in  other  cases  only  one.  As  both  these 
cases  occur  in  Artemia,  the  brine  shrimp,  and  as  the  parthenogenetic 
process  has  been  worked  out  and  understood  in  this  form  by  Brauer, 
we  shall  use  a  description  of  this  form  as  a  concrete  example. 

The  normal  number  of  chromosomes  in  the  cells  of  this  animal  is  168, 
and  that  number  exists  in  the  earlier  stages  of  the  reproductive  cells  of 
parthenogenetic  females.  At  the  time  of  maturation,  these  chromo- 
somes become  arranged  as  84  tetrads,  and  in  the  ensuing  division  these 
latter  are  separated,  84  dyads  going  to  the  first  polar  body  and  the 
other  84  staying  in  the  oocyte. 

The  subsequent  development  shows  two  forms.  In  the  second  type 
of  Brauer  (which  we  describe  first  because  it  seems  more  like  the  usual 
methods),  the  remaining  84  dyads  in  the  egg  divide,  and  one  set  of  84 
of  the  resulting  chromosomes  remains  in  place  to  form  the  egg  nucleus, 
while  the  other  84  pass  to  the  surface  to  help  form  the  second  polar  body. 

If  the  egg  nucleus  with  its  half  number  of  84  chromatin  units  were  to 
proceed  to  develop  by  cleavage,  there  would  result  an  embryo  and  adult, 
all  of  whose  nuclei  had  but  one  half  of  the  proper  number  of  chromo- 
somes. Before  development  begins,  however,  the  second  polar  body 
returns  and  unites  with  the  egg  nucleus,  and  then  development  proceeds 


FEMALE  REPRODUCTIVE   CELLS 


475 


as  though  it  were  a  male  element  that  had  been  added  to  the  egg  nucleus 
instead  of  its  own  sister  cell,  the  second  polar  body.  This  process  is 
shown  in  Figure  447,  which  is  copied  from  Brauer. 

The  other  type  of  maturation,  Brauer's  first  type,  may  be  said  to  be 
like  the  one  that  we  have  just  described  except  that  the  second  polar  body 


FIG.  447. — Artemia  salina.  Several  stages  in  the  maturation  of  the  kind  of  parthenogenetic 
egg  that  gives  off  two  polar  bodies.  A,  formation  of  first  polar  body  (p.b.2) ;  84  dyads  in  this 
polar  body  and  84  others  in  remaining  nucleus;  B,  second  division  of  the  egg  chromatin, 
which  results  in  a  second  polar  body  with  84  single  chromosomes  and  an  egg  nucleus  with  84 
chromosomes;  B,  return  of  the  second  polar  body  (p.b.2) ;  C,  D,  E,  three  stages  in  the  union 
of  the  second  polar  body  and  in  the  first  cleavage  division  of  the  completed  zygote.  (From 
WILSON  after  A.  BRAUER.) 

is  never  formed,  the  chromosomes  that  formed  it  in  the  preceding  exam- 
ple merely  remaining  in  place.  In  fact,  it  seems  a  useless  proceeding 
for  this  second  maturation  cleavage  to  take  place  in  any  event,  when  the 
chromosomes  are  to  immediately  return  and  again  join  those  from  which 
they  had  been  separated  but  shortly  before.  The  latter  method,  there- 
fore, seems  to  be  the  ultimate  specialization,  while  Brauer's  second 
method  is  a  more  primitive  process.  Figure  448,  from  the  same  source 
as  447,  shows  some  of  the  principal  stages  in  this  process. 


476 


HISTOLOGY 


Technic.  — The  technic  used  in  preparing  the  female  reproductive 
cells  for  study  differs  very  markedly  from  that  of  the  preceding  part. 
This  is  owing  solely  to  the  great  mass  of  yolk  which  most  ova  contain, 
as  well  as  to  the  membranes  of  several  kinds  with  which  they  are  usually 


o 

t;?t> 


V**j* 

b'^gftbo 


FIG.  448. — Artemia  salina.  Several  stages  in  the  maturation  of  the  kind  of  parthenogenetic 
egg  which  gives  off  but  one  polar  body.  A,  first  polar  spindle  with  84  tetrad  chromosomes; 
B,  C,  formation  of  first  polar  body;  D,  egg  nucleus,  formed  from  the  remaining  dyad  chro- 
mosomes (84);  E,  F,  G,  formation  of  first  cleavage  figure.  (From  WILSON  after  A.  BRAUER.) 

covered.  For  this  reason,  Flemming's  fluid  is  rarely  used,  and  if  a  general 
fixative  were  demanded,  we  should  first  mention  sublimate-acetic  (a  sat- 
urated, watery  solution  of  corrosive  sublimate  containing  five  per  cent 
of  glacial  acetic  acid).  Even  this  has  to  be  put  aside  in  many  cases 
where  the  yolk  becomes  too  brittle,  and  its  place  is  taken  in  most  cases  by 
a  mixture  of  picric  acid  and  acetic  acid  in  several  combinations.  In 
some  few  cases  the  yolk  can  be  softened  or  even  dissolved.  For  instance, 
by  fixing  teleost  fish  eggs  in  corrosive  sublimate  only  long  enough  to 
kill  the  embryo,  and  then  completing  the  process  in  five  per  cent  formalin, 


FEMALE  REPRODUCTIVE   CELLS  477 

the  yolk  is  usually  softened  and  may  be  cut.  Or,  by  fixing  the  ova  of 
Loligo  in  osmicacid  until  the  outer  blastoderm  is  killed,  and  then  remov- 
ing to  a  very  dilute  solution  of  chromic  acid,  the  yolk  may  be  completely 
dissolved. 

In  refractory  cases,  celloidin  combined  with  paraffin  is  used,  and  for 
some  ova  celloidin  alone,  which  makes  it  hard  to  cut  thin  enough  sections 
or  to  secure  a  good  series.  Many  studies  have  been  performed  by  smear 
preparations  with  the  same  limitations  as  have  been  mentioned  in  the 
case  of  the  male  reproductive  cells.  A  very  important  method  is  to 
study  some  ova  in  the  living  state  or  to  slice  or  tease  off  the  blastoderm 
and  mount  it  whole. 

LITERATURE 

BOVERI,  T.     "Zellen  Studien."     Jena,  1887. 

KIRKHAM,  W.  B.     "Maturation  of  the  Egg  of  the  White  Mouse,"  Trans,  of  Conn.  Acad. 

of  Arts  and  Sci.,  Vol.  XIII,  1907. 
PHILIPS,  E.  F.     "A  Review  of  Parthenogenesis,"  Proc.  Am.  Phil.  Soc.,  Vol.  XLII,  p.  174, 

1903. 
STEVENS,  N.  M.     "A  Study  of  the  Germ  Cells  of  Aphis  rosce.  and  Aphis  cenothera," 

Journ.  of  Exp.  Zool.,  Vol.  II,  1905,  p.  313. 
BRAUER,  A.     "Zur  Kentniss  der  Reifung  des  parthenogenetisch  sich  entwickelnden  Eies 

von  Artemia  salina,"  Arch.f.  mik.  Anat.,  Band  XLIII,  1893. 
LAMS  et  DOORME,  "Nouvelles    Recherches   sur   la    Maturation  et   la  Fecondation  de 

Poeuf  des  Mammiferes,"  Arch,  de  Biol.,  T.  XXIII,  1907. 
CONKLIN,  E.  G.     "  Karyokinesis  and  Cytokinesis  in  the  Maturation,  Fertilization,  and 

Cleavage  of  Crepidula,"  Journ.  Acad.  Nat.  Sci.,  Phila.  '2,  i,  1902. 
MUNSON,  J.  R.     "Researches  on  the  Oogenesis  of  the  Tortoise,  Clemmys  marmorata," 

Am.  Journ.  Anat.,  Vol.  Ill,  p.  311. 
GOLDSCHMIDT,  R.     "  Untersuchungen  liber  die   Eireifung,  Befruchtung,  und  Zellteilung 

bei  Polystomum  integrimum  Rud.,"  Zeit.  Wiss.  Zool.,  Band  LXXI,  S.  379,  1902. 
JORDAN,  H.  E.     "On  the  Relation  between  Nucleolus  and  Chromosomes  in  the  Maturing 

Oocyte  of  Asterias  forbesii,"  Pub.  Carnegie  Inst.  of  Wash.  Pub.  No.  102,  1908. 


CHAPTER   XXII 
MALE  AND   FEMALE  NIDAMENTAL  TISSUES 

BY  the  nidamental  tissues  is  meant  all  those  animal  tissues  which  are 
used  to  secrete  or  form  coverings  of  fluid  or  solid  material  in  which 
the  ripe  reproductive  elements  are  to  be  transported  or  protected  upon 
leaving  the  gonads.  Many  forms  show  no  development  of  such  organs, 
and  the  eggs  and  sperm  are  cast  out  at  random  into  the  surrounding 
water.  The  sea  urchin  and  starfish  show  an  example  of  this  condition. 
The  degree  of  the  individual's  specialization  and  organization  does  not 
seem  to  affect  the  development  of  these  structures,  as  can  be  noticed 
when  we  recall  that  most  of  the  highly  organized  and  specialized  teleost 
fish  deposit  their  ova  and  sperm  much  as  the  echinoderms  do.  Also 
many  low  and  simply  organized  creatures  have  elaborate  nidamental 
organs,  as  the  flatworms  and  others.  We  shall,  therefore,  in  our  dis- 
cussion, pay  little  attention  to  the  systematic  position  of  our  example, 
merely  indicating  the  conditions  under  which  it  lives  and  which  make 
these  structures  a  benefit. 

The  use  of  fluid  as  a  carrying  body  is  one  that  greatly  aids  the  proper 
placing  of  the  reproductive  cells.  While  this  is  used  in  the  discharge  of 
some  eggs,  it  is  of  especial  use,  and  a  necessity  to,  the  transportation 
of  the  very  small  spermatozoa. 

Most  eggs  are  carried  or  aided  in  their  passage  from  the  body  by : 
first,  some  sort  of  follicle  liquor  which  is  secreted  by  the  cells  of  the 
membrana  granulosa,  a  region  of  differentiated  nurse  cells  in  the  ovary 
of  man  and  other  vertebrates ;  and,  second,  by  the  general  coelomic  fluids 
of  the  body  cavity  which  occur  in  all  forms  in  which  the  gonads  rupture 
into  this  space. 

Likewise,  the  spermatozoa  are  invariably  borne  in  a  liquid  that  is 
secreted  or  in  some  way  produced  in  the  seminal  lobule.  This  liquid 
is  reenforced  in  some  cases  by  the  products  of  certain  accessory  glands 
which  pour  out  a  heavy  fluid  that  is  specially  adapted  in  consistency 
to  carry  and  discharge  the  sperm,  and  is  also  fitted  by  its  composition  to 
nourish  and  keep  them  in  health  and  activity  for  long  periods. 

The  prostate  gland  of  the  mammals  is  a  specific  example  of  such  an 
accessory  structure.  Figure  449  shows  a  section  of  this  gland  taken 

478 


NIDAMENTAL    TISSUES 


479 


con. 


from  man.    This  section  shows  a  compound  alveolo-tubular  gland  with 

a  simple  columnar  secreting  epithelium.    The  cells  of  this  layer  are  clear 

and  secrete   continu- 

ously   without    a 

degeneration  and  re- 

newal of  their  cyto- 

plasm.  The  secretion 

is  not  plainly  visible 

at    any   stage    of   its 

elaboration,    and   no 

trophospongia     have 

been  described.    The 

nucleus  is  round  and 

placed    close   to    the 

proximal  end.    Many 

concretions  are  found 

in  the  lumen  of  this 

gland. 

Another   form  of 
male-carrying 

is  Secreted  by  the  Sper-        neli   sec.,   coagulated   secretion.      (After   LEWIS,    in   "  Stohr's 

matophoral  glands  of      Text-book  °f  Histolo^"> 

certain  crustaceans,  as  the  lobster  and  the  crayfish.  This  fluid  is 
secreted  by  the  walls  of  the  sperm  ducts  (Fig.  450),  and  it  not  only 
serves  as  a  vehicle  to  carry  the  mass  of  sperm  out  of  the  male  organs, 
but  it  also  forms  a  semifluid  covering  around  them  and  attaches 
itself  to  a  receiving  plate  on  the  female  body  and  hardens,  preserving 

the  life  of  the  spermatozoa  for 
months  or  even  years  until  they  are 
needed  to  fertilize  the  eggs.  When 
this  time  comes,  the  female  surface 
secretes  a  fluid  which  softens  the  hard- 
ened sperm  fluid  and  brings  the  dor- 
mant spermatozoa  back  to  activity. 
Such  a  package  is  known  as  a  sper- 
matotheca. 

Other    Carrying    fluids    f  Of   the    SpCf- 


ntus.f. 


a    .,    FIG.  449. — Part  of  several  acini  of  the  human  prostate  gland. 
fluid        cow.,  concretion;  mus.f.,  smooth  muscle  fibers;  bl.,  blood  chan- 


FIG.  450.  —  Transection  of  the  seminal 


»-*»«    «»    to    be   found   in   many 
sticky,  spermathecal  covering  for  the    other    animal    forms.     A   step   in   the 
toicK.)  spermat°z°a'     (After  H"    organization  of  this  apparatus  may  be 
seen  in  the  salamander,   the  male  of 

which  secretes  a  covering  for  the  sperm.     This  takes  place  in  the 
thickened  folds  of  integument  which  border  upon  the  cloacal  open- 


48o 


HISTOLOGY 


ing.     Some  leeches  also  form  double  spermatotheca  as  described  by 

Whitman. 

The  most  complicated,  exact,  and  highly  specialized  of  the  male 

nidamental-tissue  products  is  to  be  seen  in  the  spermatophores  of  certain 
cephalopod  mollusks.  This  beautiful  mechan- 
ism is  produced  in  a  differentiated  portion  of 
the  seminal  duct.  This  differentiated  region 
consists  of  four  divisions  of  the  seminal  tube. 
A  prostate  gland  described  in  an  unpublished 
work  by  L.  W.  Williams  as  a  compound  lamel- 
lar gland,  and  a  glandular  caecum  take  part  in 
the  formation  of  the  spermatophore.  The  exact 
manner,  however,  in  which  the  structure  is 
formed  is  not  known.  Figure  451  gives  some 
\n|  idea  of  its  complexity  and  of  the  delicacy  of  the 

processes  by  which  it  was  formed. 

In  the  female,  the  number,  complexity,  and 
peculiar  distribution  of  nidamental  structure  is 
a  formidable  obstacle  to  a  short  and  compre- 
hensive account  of  them.  As  has  been  said, 
some  comparatively  few  forms  discharge  the 
eggs  externally  in  a  fluid.  And  again,  some  of 
the  highest  forms  which  develop  the  ova  inter- 
nally, do  so  without  the  aid  of  any  shell  or  fluid 
envelopes  whatever.  The  follicular  and  ccelo- 
mic  discharging  fluids  have  already  been  men- 
tioned in  the  remarks  on  that  subject. 

The  planarian  worms  make  cocoons  in  the 
uterus  by  depositing  a  chitinous  envelope 

FIG.  451.  —  View  of  the  central  around  the  ovum.     In  some  forms  this  cocoon 


The  mass  of  spermatozoa  by  which  it  is  attached,  as  a  stalk,  to  rocks, 


trate  the  exact  and  finished  these  cocoons  are  secreted  by  the  columnar  epi- 


Among  some  other  worms  can  be  found 
fairly  well  specialized  nidamental  tissues.  Two  of  these,  the  earth- 
worm, Lumbricus,  and  the  leech,  Pisicola,  show  interesting  and  typical 
forms. 

The  specific  cells  of  both  of  these  organs  are  modified  mucous  cells, 
which  have  probably  been  evolved  from  the  ordinary  epithelial  mucous 
cells,  such  as  are  to  be  seen  in  the  covering  epithelium  of  the  earthworm. 

The  nidamental  cells  of  Pisicola  lie  inside  the  body  cavity,  or,  to  be 


NIDAMENTAL    TISSUES 


481 


more  exact,  they  form  two  longitudinal,  lateral  bands  just  inside  the 
longitudinal  musculature  of  the  body  wall.  The  cells  are  enormous 
in  size  and  very  roughly 
cubical  in  shape.  They 
lie  with  what  must  be 
considered  their  proxi- 
mal end  against  the  lon- 
gitudinal muscle,  and, 
although  they  are 
packed  very  closely  in 
the  layer,  there  is  al- 
ways a  space  between 
them  for  the  access  of 
blood  or  ccelomic  fluid. 
A  fine,  thin  connective 
tissue  with  small, 
highly  differentiated 
cells  surrounds  the  cell 
body  (Fig.  452). 

The     nucleus     lies 
somewhat       proximal, 

.    r               .  FIG.  452.  —  Gland  cell  from  leech,  Pisicola.    sec.,  secreted  ma- 

and  IS  Very  irregular  in  terials  in  various  stages  of  elaboration;  ncl.,  nucleolus;  nu., 

shape.      It  is  drawn  OUt  nucleus;  cyt.ch.,  cytoplasmic  channels  containing  and  deliv- 

,             f         .  ering  the  secretion    granules   to  the   large   distal  vacuole; 

at  a  number  Ot   points  #.».,  discharging  tubes  of  this  and  two  other  cells. 

on     its     surface     into 

irregular  and  thin,  pointed  processes.  The  chromatin  appears  as  a 
large  number  of  granules,  and  there  are  evidently  other  bodies  in  it 
• — an  achromatic  nucleolus  and  some  roughly  rod-like  and  pointed  chro- 
matin bodies. 

The  cytoplasm  is  of  most  interest.  Proximally,  it  is  rather  more 
homogeneous,  but,  distally,  in  the  cell  body  its  place  is  almost  entirely 
taken  by  the  great  secretion  vacuole.  Reaching  back  from  this  vacuole 
are  a  series  of  branching  channels  or  trophospongia,  and  in  the  cytoplasm 
which  borders  on  the  channels  can  be  seen  secretion  granules  in  all 
stages  of  formation.  When  completed,  these  granules  are  discharged 
into  the  channels  and  carried  down  into  the  large  secretion  chamber  or 
vacuole. 

This  vacuole  is  produced  distally  into  a  long  tube,  which  runs,  together 
with  a  group  of  similar  tubes  from  other  cells  of  this  kind,  anteriorly  to 
a  region  near  the  head,  where  all  these  tubes  penetrate  the  body-wall 
tissues,  and  end  externally  between  the  columnar  epithelium  cells  of 
the  epidermis.  The  secretion  is  mucin,  and  although  the  cells  have  been 
pointed  out  as  excretory  cells,  there  is  no  doubt  that  they  are  mucous 

21 


482 


HISTOLOGY 


cells  of  the  epidermis,  extraordinarily  developed  and  enlarged  to  make 
a  cocoon  for  the  eggs.  A  considerable  proportion  of  the  body  is  com- 
prised by  these  cells,  and  the 
expenditure  of  the  animal's 
energy  in  secreting  mucus  must 
be  large. 

The  secretion  appears  in 
some  of  the  cells  in  a  dis- 
solved state,  and  as  solid 
granules  in  others.  It  is  used 
both  to  make  the  cocoon  and 
to  keep  the  body  covered  with 
the  slime  that  is  always  found 
on  its  surface.  The  making 
of  the  cocoon  has  never  been 
described  or  seen  by  the  writ- 
ers in  this  species,  which  was 
taken  from  summer  flounders 
on  the  Massachusetts  coast, 
but  it  is  undoubtedly  done  by 
forming  a  shell  of  mucus  in 
the  clitellar  region  around  the 
body,  and  then,  after  deposit- 
ing the  eggs  and  sperm,  slip- 
ping it  off  the  body  and  at- 
taching it  as  other  leeches  do. 
The  nidamental  mucous 
tissues  of  the  earthworm  are 
to  be  found  in  an  elevated, 
band-like  ring  about  the  an- 
terior part  of  the  body,  the 
clitellum.  Transverse  sections 
of  this  region  show  that  mucus 
is  secreted,  not  from  unicel- 
lular glands  as  it  is  in  the  rest 
of  the  animal's  body,  but 
from  closely  set,  tubular  in- 
vaginations  in  whose  long 
acini  can  be  seen  the  large 
mucus-forming  cells  (Fig. 
453).  The  cells  do  not  show  a  very  clearly  marked  innate  type  at 
the  bottom  of  the  fundus.  For  this  reason,  and  also  because  their 
nuclei  show  no  degeneration  process,  it  may  be  concluded  that  they 


FIG.  453.  —  Distal  (.4)  and  proximal  (B)  portions  of 
a  single  clitellar  gland  of  the  earthworm,  Lumbri- 
cus.  X  780. 


NIDAMENTAL    TISSUES  483 

are  permanent  cells  and  not  destroyed  in  the  process  of  forming  the 
secretion. 

The  real  cytoplasmic  body  of  these  gland  cells  is  hard  to  define  on 
account  of  the  large  amount  of  secreted  material  with  which  the  cells 
are  filled.  The  nucleus  lies  proximal  in  the  cell  and  is  large,  and  usually 
round.  Its  plasmosome  is  larger  than  any  other  found  in  the  tissues  of 
Lumbricus  except  in  the  young  ova  and  in  some  nerve  cells.  Toward 
the  gland  mouth  and  lining  the  exterior,  the  nuclei  are  somewhat  smaller 
and  inclined  to  be  oval.  The  distal  part  of  the  cell  body  is  bent  up  and 
extends  up  through  the  lumen  toward  the  mouth  of  the  gland.  To- 
gether with  the  same  parts  of  its  fellow-cells,  it  fills  the  lumen,  and  one 
cannot  determine  easily  where  the  cell  ends  distally.  The  secreted 
material  is  very  clear,  and,  if  granular,  is  very  finely  so.  In  the  peripheral 
part  of  the  gland  the  secretion  is  more  coarsely  granular. 

That  part  of  the  epithelium  which  touches  the  exterior  is  entirely 
different  in  appearance.  The  cells  are  very  narrow,  and  their  nuclei 
are  also  long,  narrow  ovals  in  shape.  The  nucleus  has  a  more  abundant 
supply  of  chromatic  material,  and  the  plasmosome  is  smaller  than  the 
nucleus  of  the  deeper  cells.  These  cells  also  secrete,  but  the  material 
is  not  so  abundant. 

Very  highly  developed  female  nidamental  tissues  are  to  be  found 
among  the  mollusks.  — They  are  used  to  make  both  individual  and  col- 
lective envelopes  for  the  ova.  One  especially  interesting  one  is  to  be  seen 
in  the  gasteropod,  Sycotypus  canaliculatus,  in  which  the  egg  case  is  formed 
in  a  heavy-walled,  glandular  part  of  the  oviduct.  The  tough  and  mem- 
branous walls  of  the  many  egg  cases,  as  well  as  the  string  to  which  they 
are  regularly  attached,  are  all  secreted  by  the  glands  found  in  the  walls 
of  the  oviduct,  a  long  tube  which  carries  the  eggs  to  the  exterior. 
The  eggs  come  into  the  oviduct  in  groups  of  from  40  to  150, 
in  a  medium-sized  specimen.  Each  group  lies  in  a  fold  in  the  thick, 
glandular  wall,  and  this  fold  forms  a  case  around  it.  The  glandular 
thickness  occupies  two  longitudinal  bands  of  the  lining  of  the  duct,  and 
on  one  side,  where  they  meet,  is  a  long  groove  in  which  the  string 
that  bears  the  cases  is  formed  in  situ  with  the  cases  attached  to  it.  The 
epithelium  lining  this  groove  is  different  from  that  which  secretes  the 
materials  for  the  cases.  Figure  454  is  a  rough  diagram  of  one  side  of  the 
groove,  and  a  small  portion  of  the  glandular  thickening  on  the  same 
side. 

At  the  bottom  of  the  groove  it  is  least  differentiated  and  is  a  columnar 
epithelium  with  rather  narrow  cells  and  nuclei.  On  the  lower  side  of 
the  groove  it  begins  to  be  thrown  into  folds  and  an  occasional  large  "gob- 
let cell"  full  of  mucin  granules  is  seen,  especially  on  the  top  of  the  folds. 
This  condition  is  exaggerated  nearer  the  top  of  the  groove,  where  the 


484 


HISTOLOGY 


folds  of  epithelium  are  much  higher ;  the  cells  on  the  high  curve  are  all 
provided  with  mucous  vacuoles. 

Another  feature  is  to  be  noted  here.  Besides  possessing  goblet  cells, 
the  epithelium  on  the  folds  is  evaginated  into  long  tubular  glands,  the 
first  of  which  are  unicellular  glands  with  their  basal  portions  lying  in 
the  connective  tissue  beneath  the  epidermis.  These  are  rapidly  sup- 
planted farther  out  of  the  groove  by  multicellular  glands  which  open 


FIG.  454.  —  Outline  sketch  of  one  side  of  the  groove  in  a  transverse  section  of  the  oviduct  of 
Sycotypus  canaliculatus .  To  the  right  are  seen  the  simple  folds,  with  occasional  unicellular 
mucous  cells.  In  the  middle  the  heavier  folds  with  both  unicellular  and  multicellular  glands. 
To  the  left  the  very  large  multicellular  glands  with  dotted  outlines  to  mark  the  regions  fig- 
ured in  the  next  illustration.  X  50. 


by  narrow  ducts,  and  whose  lumen  is  filled  with  a  blue  staining  content 
which  still  shows  mucin  reactions. 

Out  on  the  primary  nidamental  surface,  the  same  structural  condi- 
tions obtain,  except  that  they  are  exaggerated,  so  far  as  the  glands  are 
concerned.  These  are  much  lengthened,  and  their  straight,  parallel 
bodies  form  a  huge  layer,  which  gives  this  part  its  swollen,  whitish  ap- 
pearance. Two  of  these  glands  are  represented  to  the  left  of  Figure  454. 
As  in  the  earthworm's  clitellum  the  cells  are  best  defined  at  the  fundus, 
and  farther  distally  their  outer  bodies  cannot  be  distinguished  in  the  mass 
of  secretion  which  fills  the  lumen.  The  proximal  portions  of  these  glands 


NIDAMENTAL    TISSUES 


485 


form  an  almost  solid  mass  with  but  little  connective  tissue  between  them, 
and  the  gland-cell  nuclei  lie  flat  in  the  cells.  On  the  other  hand,  the 
distal  parts  do  not  lie  so 
close  to  one  another,  but 
are  separated  by  a  consid- 
erable space  filled  with  the 
typical  molluscan  connec- 
tive tissue  with  its  coarse 
reticulum  and  many  alveoli. 
In  the  figure  it  can  be  seen 
that  blood  channels  occur 
in  these  spaces.  The  se- 
creting cytoplasm  of  the 
gland  cells  shows  a  deli- 
cate reticulum  with  fine, 
black  granules  at  the  inter- 
sections of  its  strands,  and 
large,  light-blue  staining 
granules  in  the  meshes. 
These  latter  are  probably 
the  secretion;  the  first  are 
possibly  microsomes. 

The  opening  of  these 
glands  is  exceedingly  small 
and  hard  to  see,  among 
the  epithelial  cells  on  the 
primary  surface.  Figure 
455  shows  the  single  distal 
part  and  double  fundus  of 
one  of  these  glands  as  indi- 
cated by  the  dotted  lines 
on  Figure  454.  The  cells 
lining  the  surface  outside 
of  the  groove  show  no  mucin  inclusions,  and  have  very  large  basal 
granules  on  the  marginal  ends  of  their  short,  strong  cilia.  These 
cilia  are  used  to  move  the  eggs  and  egg  cases,  there  being  no  peri- 
stalsis. 

Among  the  vertebrate  animals  we  shall  examine  the  histological  struc- 
ture of  the  nidamental  organs  of:  first,  a  urodele  amphibian  that  forms 
two  jelly  coverings  for  its  individual  ova;  second,  a  teleostfish  that  forms 
a  single  jelly  covering  for  a  great  many  of  its  eggs;  third,  a  selachian  fish 
that  forms  an  albumen  covering  followed  by  a  tough,  chitinous  covering 
for  each  of  its  eggs;  and,  lastly,  a  bird  which  forms  a  tough  albuminous 


FIG.  455.  —  Distal  and  proximal  extremities  of  a  com- 
pound tubular  nidamental  gland  from  the  oviduct  of 
Sycotypus  canaliculatus .  The  actual  opening  of  the 
duct  is  small  and  obscure,  and  its  location  is  indicated 
in  the  figure  by  the  depression  in  the  ciliated  epithe- 
lium. X  500. 


486 


HISTOLOGY 


layer,  a  soft  albuminous  layer,  two  membranous  layers,  and  finally  a 
hard  outer  shell  for  each  egg. 

The  first  type  of  organ  mentioned  will  be  well  represented  by  the  ovi- 
duct of  the  salamander,  Desmognathus  fusca.  The  specimen  represented 
in  Figure  456  was  killed  a  short  time  (a  few  weeks)  before  the  breeding 
season,  and  consequently  the  nidamental  tissues  were  preparing  for  their 
task  by  an  increase  in  the  size  and  characters  of  their  specific  cells. 

This  tissue  consists  of  a  long  tube  with  an  expanded  open  end  that 
lies  far  forward  in  the  body  cavity.  From  this  end  it  takes  a  curving 
course  to  the  point  at  which  it  empties  into  the  cloaca.  The  ova  enter  its 
upper  end  by  the  action  of  ciliated  cells,  and,  while  on  their  way  down  its 
lumen,  each  one  is  invested  by  two  coats  of  a  jelly  substance.  These 


FIG.  456.  —  Transaction  of  an  oviduct  of  the  salamander,  Desmognathus  fusca.     x  80. 

coats  are  not  as  thick  as  they  would  be  in  a  frog  or  the  salamander, 
Amblystoma.  The  outer  coat  is  thinner  and  tougher  than  the  inner. 
The  inner  layer  is  put  on  the  egg  in  the  upper  part  of  the  duct,  but  no 
structural  differences  are  to  be  noted  between  the  glands  of  this  part 
and  those  of  the  point  at  which  the  tougher  coat  is  applied.  Such  a 
difference  must  exist,  but  it  may  be  a  chemical  difference  or  one  not 
easily  noted  in  the  structure  of  the  cells. 

Figure  456  shows  a  section  of  the  tube  near  its  upper  end,  or  where 
the  first  jelly  layer  is  applied.  The  large  lumen  of  the  duct  is  closed,  of 
course,  in  this  specimen,  owing  to  the  fact  that  it  was  not  distended  either 
naturally  or  artificially  by  any  fluids. 

Opening  into  this  lumen  are  a  great  many  wide  tubular  glands  which 
show  a  tendency  to  become  larger  or  even  compound  at  their  proximal 


NIDAMENTAL    TISSUES  487 

(outer)  ends.  In  transverse  sections  of  the  oviduct  these  tubular  glands 
have  been  mistaken  for  sections  at  right  angles  to  longitudinal  corruga- 
tions. The  right-hand  part  of  our  figure,  although  a  transverse  section, 
gives  proof  that  such  is  not  the  case  in  this  form,  and  that  the  folds 
in  the  section  represent  glands  of  a  tubular  variety. 

The  cells  lining  these  glands  form  a  columnar  epithelium,  and  are 
intensely  active  in  secreting  the  jelly  substance.  The  secretion  appears 
as  a  granular  mass  that  fills  their  distal  ends.  The  nuclei  lie  flattened 
against  the  proximal  surface,  and  are  very  small  and  dense.  The  outer 
edges  of  the  folds  represent  the  primary  surface  of  the  duct  lumen. 
Here  the  gland  cells  give  way  to  a  non-secreting  form  with  a  larger 
nucleus  placed  in  the  middle  height  of  he  cell. 

The  whole  tube  is  surrounded  by  an  inner  and  an  outer  longitudinal 
layer  of  smooth  muscle.  The  ovum  is  passed  through  the  tube  by  a 
wave-like  contraction  of  these  layers,  which  is  called  peristalsis. 

The  fish,  Pterophryne  hisirio,  is  one  of  a  large  group,  the  pediculate 
fishes,  which  lay  their  eggs  embedded  in  a  long  ribbon-shaped  plate  of 
jelly  which  floats  on  the  sea  until  the  eggs  hatch. 

This  ribbon  of  jelly  is  made,  not  in  an  oviduct,  for  the  bony  fishes 
have  no  long  oviduct,  but  by  a  part  of  the  ovary  itself.  This  organ,  like 
that  of  other  teleosts,  is  a  tube-like  sac  formed  by  the  union  of  two  epithe- 
lial folds.  In  a  mackerel  and  in  most  other  members  of  this  group  the 
entire  inner  surface  of  this  long  pocket  is  used  to  produce  the  ova.  In 
Pterophryne  the  sac  hangs  from  its  suspensory  membrane,  with  its  lumen 
closed  to  a  straight  line  and  its  cylindrical  wall  thereby  divided  into  two 
halves  which  rest  against  each  other,  face  to  face.  One  side  is  used  to 
develop  the  ova,  and  this  side  is  raised  into  the  numerous  evaginated  pa- 
pillae that  occur  in  the  other  teleost  ovaries.  The  other  and  opposite  side 
retains  its  simple  epithelial  structure,  and  the  epithelium  secretes  the 
jelly  materials  in  the  form  of  a  flat  sheet,  the  exact  length  and  width  of 
the  layer  from  which  it  originated.  The  jelly  strip  and  the  ova  are  pro- 
duced at  the  same  time,  and  their  position,  opposite  and  against  each 
other,  causes  the  ova  to  be  pressed  into  the  jelly,  and  then  the  whole 
mass  is  passed  out  of  the  ovary,  one  from  each  of  the  two  divisions,  and 
the  jelly  swells  a  little  and  floats  about  until  the  eggs  develop.  Figure 
457  represents  a  fold  (not  a  gland)  of  this  membrane  in  section.  The 
skate,  Raja  erinacea,  will  serve  to  represent  the  third  vertebrate  type  to 
be  described,  in  which  the  ovum  is  covered  first  with  a  coat  of  jelly-like 
albumen,  and  afterward  with  a  tough  shell  of  rather  remarkable  shape. 
As  in  the  amphibian,  the  ovum  is  set  free  from  the  ovary  into  the  body 
cavity  and  is  then  conducted,  probably  by  ciliary  motion,  into  the  opening 
at  the  anterior  end  of  the  tube-shaped  oviduct.  This  wide  opening  soon 
narrows  down,  and  in  its  upper  part  the  albumen  covering  is  secreted 


HISTOLOGY 


by  a  columnar  epithelium  which  lines  the  wide  lumen  without  any  am- 
plification whatever,  as  was  the  case  in  other  oviducts.  The  cells  are 

columnar  and  have  both  a 
thickened  border  and  a  line 
of  cilia.  The  secretion  ap- 
pears as  a  globule  in  the 
cell  body,  and  is  discharged 
as  mucin  is. 

A  short  distance  below, 
the  tube  narrows  somewhat, 
and  its  walls  become  thicker. 
This  region  is  not  extensive, 
and  forms  a  short  but  promi- 
nent enlargement  called  the 
shell  gland.  An  examina- 
tion of  the  epithelium  of 
this  organ  shows  that  it  is 
lined  by  much  the  same  kind 
of  epithelium,  except  that 
the  cells  are  much  larger  and 
the  surface  is  thrown  into 
shallow  tubular  glands 
whose  cells  have  undergone 
no  marked  differentiation, 
and  which  are  probably  ob- 
literated by  stretching  when 
the  large  egg  passes  through. 
In  the  lowest  part  of  the 
oviduct,  the  walls  are  much 
thicker,  the  lumen  larger, 
and  the  epithelial  lining  is 
changed  from  the  columnar 

FIG.  457- -Parts  of  the  opposed  epithelia  in  the  ova-     f°rm  °f  the  Feeding  parts 
rian  tube  of  the  fish,  Pterophryne  histrio;  d.s.,  distal     to    a    very    peculiar,    distally 
surfaces  of  the    two  epithelia;    nid.ep.,  nidamental 
epithelium;    rep.ep.,  reproductive  epithelium  in  out- 
line; ov.,  outlines  of  two  ova  lying  in  follicles  inside 
the  reproductive  epithelium,     x  840. 

as  a  ribbon-like  fiber  for  almost  halfway  out  to  the  lumen.  The  outer 
cells  are  wedged  in  between  the  ends  of  basal  cells  and  form  a  somewhat 
irregular  distal  boundary  to  the  epithelium.  Here  they  show  signs  of 
degeneration.  The  possibility  remains  that  some  of  the  cells  extend 
from  basement  membrane  through  to  distal  surface,  and  that  the  layer 
is  theoretically  a  simple  epithelium.  Even  were  this  so,  the  practical 


elongate,  stratified  form  (Fig. 
458).  A  cell  of  the  basal  layer 
of  this  epithelium  extends 


NIDAMENTAL    TISSUES 


489 


result  .of  having  the  cell  bodies  with 
their  contained  nuclei  at  so  many  lev- 
els, would  be  to  make  it  serve  as  a  strati- 
fied epithelium. 

The  oviduct  of  the  fowl  acts  very 
much  as  that  of  the  skate  did.  —  It 
differs  functionally  from  that  of  the 
skate  in  producing  more  various  and 
more  numerous  coverings  for  the  egg. 
We  shall  compare  its  structure  briefly 
with  that  of  the  skate  by  describing  the 
epithelial  lining  of  an  upper  and  a  lower 
region. 

A  section  taken  near  the  beginning 
(Fig.  459)  shows  that  here  the  tube  is 
lined  with  a  columnar  epithelium  which 
is  simple,  but  whose  nuclei  are  placed 
at  somewhat  various  heights  in  the 
cells.  This  gives  a  result  which  may  be 
compared  with  the  strange  epithelium 
found  in  the  skate's  lower  oviduct.  The 
nuclei  are  found  only  distally  in  the 
layer,  which  is  exceptional.  The  cells 
show  no  signs  of  secretion,  which  may 
be  accounted  for  by  the  fact  that  the 
hen  was  not  laying  at  the  time  she  was 
killed.  A  section  taken  in  the  lower 
part  of  the  tube  shows  a  decided  differ- 
ence from  the  skate's  corresponding  tis- 
sue (Fig.  460).  This  region,  besides  its 
simple  epithelium,  is  invaginated  into 
numerous  glands  whose  ducts  and 
mouths  are,  for  some  unknown  reason, 
very  difficult  to  see.  This  was  also  the 
case  in  other  nidamental  glands,  as  the 
salamander  and  the  invertebrate  forms, 
and  seems  to  be  a  characteristic  of  this 

tissue.  FIG.    458.—  Raja    erinacea;     oviduct. 

The  glands  are  lined  by  a  cuboidal      Thick.  stratified  epithelium  whose 

.  t     i.  ,  -,    .  .  ,         cells  are  distally  elongate.     X  450. 

epithelium  whose  nuclei  are  round,  and 

whose  lumen  but  rarely  shows  in  our  specimen.  It  would  probably 
show  better  in  a  laying  hen.  The  nuclei  of  the  superficial  cells  are  long 
and  narrow;  more  so  than  in  the  same  kind  of  cells  from  the  first  region. 


490 


HISTOLOGY 


FIG.  459.  —  Callus  domesticus;  upper  part 
of  oviduct.  Shows  a  simple,  tall,  colum- 
nar epithelium.  X  1 1  oo. 


The  study  of  organs  of  copulation 
is  principally  a  morphological  one. 
The  histology  of  these  structures  is 
very  generally  that  of  mere  integu- 
ment muscle  and  other  general  tis- 
sues. 

We  shall  examine  one  case  of  a 
tissue  specifically  designed  to  aid 
this  process  in  the  mammals,  the 
erectile  tissue.  The  urethral  portion 
of  the  human  corpus  cavernosum  will 
represent  the  tissue  which  is  used 
to  enlarge  and  make  rigid  the  intromittent  organ  (Fig.  461). 

The  tissue  consists  of  a  con- 
nective tissue  containing  many 
smooth  muscle  cells.  It  is  en- 
tered by  arteries  whose  walls 
show  peculiar  elastic  tissue 
enlargements  directly  under  the 
intima.  These  arteries  end,  in 
the  cavernous  tissue,  in  capil- 
laries which  in  turn  discharge 
their  contents  into  a  series 
of  thin-walled  veins  which 

are   so  large  and  so  numerous  that  they  give  the 
the  cavernous  tissue. 

These  cavernous  veins  are  filled  with 
blood,  and  the  efferent  veins  are  so  com- 
pressed at  the  same  time  that  this  blood 
cannot  escape.  This  produces  the  neces- 
sary rigidity  of  the  tissue. 

Technic.  —  The  different  nidamental 
tissues  require  a  variety  of  treatment. 
Many  of  them  must  be  sectioned  serially 
to  work  out  the  small  and  complicated 
passages,  glands,  etc.  The  combined  cel- 
loidin  and  paraffin  method  is  most  useful 
here,  especially  when  the  tissue  is  stained 
in  bulk.  In  order  to  insure  a  series  un- 
broken by  stiff  and  curling  sections,  it  is 

FIG.  461.  —  Small  area  from  a  sec-       ,  3 

tion  through  the  corpus  cavemo-     best  to  use  a  very  weak  celloidm  mixture, 
sum  of  an  infant,   v.,  vein;  art.,  Manv  of  the  nidamental  structures  will 

artery;     mus.f.,    smooth    muscle       ,        .  .    ,    .         .        ,  i        -11 

fibers,    x  500.  be  found  lying  in  the  passages  and  will 


FIG.  460.  —  Callus  domesticus;  lower  region  of  ovi- 
duct with  short  tubular  glands.     X  noo. 


tissue  its  name, 


mus.  f. 


NIDAMENTAL    TISSUES  491 

offer  obstructions  that  are  not  easily  surmounted.  Much  can  be  done 
to  help  this  by  seeing  that  these  are  removed  by  natural  processes  at 
the  time  of  killing  the  animal.  The  secreted  material  in  the  cells 
themselves  often  makes  the  sections  brittle  and  poor. 

A  better  appreciation  of  these  tissues  may  be  obtained  by  observing 
them  in  action  during  the  life  of  the  animal.  Linton  and  Curtis  have 
studied  them  thus  in  transparent  worms,  and  much  interesting  work  has 
been  done  in  this  way.  Such  observations  have  been  made  on  larger 
animals  which  were  not  transparent,  as  the  cephalopod  mollusks,  by 
cutting  out  or  exposing  the  parts  in  a  freshly  killed  animal  and  watch- 
ing the  operations  that  were  automatically  performed  until  near  the 
death  of  the  tissue,  which  takes  place  some  time  after  that  of  the  individ- 
ual as  a  whole. 

LITERATURE 

ELLERMANN.     "tiber  die   Schleimsekretion   in   Eileiter  der  Amphibien,"   Anat.   Am., 

Band  XVIII,  1900. 

SCHNEIDER.     "Monographies  der  Nematoden."     Berlin,  1866. 
ANDREWS,  E.  A.     "Habits  of  the  Crayfish,"  Johns  Hopkins  Univ.  Circular,  Vol.  XIV, 

p.  74. 
BROCK,  J.     "Uber  die  Geschlechtorgane  der  Cephalopoden,"  Zeits.f.  Wiss.  Zool.,  Band 

XXXII,  1879. 
LINTON,  E.     "  Fish  Parasites  collected  at  Woods  Hole  in  1898,"  Bull,  of  the  U.  S.  F.  C., 

1899,  pp.  267-304. 
CURTIS,  W.  C.     "The  Life  History,  the  Normal  Fission,  and  the  Reproductive  Organs  of 

Planaria  maculata."     Proceedings    of  the  Boston  Society  of  Natural  History,  Vol. 

XXX,  No.  7,  1902. 
WILLIAMS,  L.  W.     An  unpublished  article  on  the  anatomy  of  Loligo  Pealii  about  to 

appear  in  the  Bull,  of  the  U.  S.  F.  C. 


CHAPTER    XXIII 
PARENTAL   AND    EMBRYONIC   NOURISHING   TISSUES 

MANY  organisms  cast  their  ova  out  into  the  surrounding  water,  to 
develop  and  perpetuate  the  race  by  sheer  chance  and  force  of  numbers. 
Others  place  them  in  "nests,"  and  some  of  these  even  go  about  with  the 
young  for  some  time  afterward,  aiding  them  by  their  protection  from 
enemies  and  in  their  search  for  food.  The  parent  may  even  bring  them 
food  until  they  can  procure  it  themselves.  Still  another  class  of  parents 
feed  the  young  both  before  and  after  birth  with  some  product  or  part  of 
their  own  body.  It  is  with  the  tissues  whereby  they  perform  this  latter 
function,  that  this  chapter  deals. 

The  most  frequent  method  of  supporting  the  young  is  inside  the 
body,  in  contact  with  some  surface  through  which  the  food  can  be  passed 
and  the  waste  products  returned  to  the  parent's  blood.  This  process 
probably  had  its  origin  in  many  of  the  organisms  which  kept  the  young 
in  the  body  for  development  on  then-  egg-yolk  supply.  Such  a  method 
may  be  observed  in  hundreds  of  forms,  some  of  them  very  lowly  organized. 
Sometimes  the  attachment  to  the  parent's  body  is  maintained  through 
the  membranes  of  the  mother's  or  father's  mouth,  as  in  a  catfish,  As- 
predo  batrachus,  or  the  skin  of  the  back  or  belly  as  in  a  toad,  Pipa 
Americana.  This  external  connection  possibly  furnishes  the  young  with 
some  benefits  in  its  growth,  possibly  some  small  amount  of  fluid  food. 

The  spiny  dogfish,  Acanthias  vulgaris,  is  one  of  the  animals  which 
retain  the  ova  in  the  reproductive  passages  during  development.  — The 
egg  is  very  large,  and  when  discharged  from  the  ovary  it  passes  into  the 
oviduct,  and  in  the  upper  part  of  this  tube  it  is  provided  with  a  watery 
jelly  layer  and,  later,  with  an  outer  covering  of  thin,  transparent,  flexible 
material  which  easily  ruptures,  but  which,  in  other  sharks  and  rays,  is 
tough  and  heavy. 

When  the  egg  is  developed  so  that  the  young  animal  is  of  some  size, 
this  outer  shell  breaks,  and  the  young  animal  lies  in  the  dilated  lower 
portion  of  the  oviduct,  which  thus  becomes  the  uterus.  Here  it  lies  in  a 
fluid,  and  the  walls  of  the  uterus  become  specialized  to  present  a  larger 
blood  supply  and  a  greater  surface  for  exchanges  between  the  blood 
and  the  uterine  fluid. 

492 


NOURISHING  MEMBRANES 


493 


With  the  naked  eye,  it  can  be  seen  that  the  entire  inner  surface  of  the 
uterus  is  thrown  into  a  series  of  flat  circular  flaps,  or  papillae,  and  a  good- 
sized  blood  vessel  can  be  seen  passing  along  the  edge  of  each  flap.  The 
true  line  of  junction  of -the  flap  to  the  wall  is  always  longitudinal,  and  it 
can  be  seen  at  a  glance  that  this  surface  amplification  affords  more  than 
double  the  original  surface  of  the  cavity. 

Sections  taken  transversely  to  the  uterus,  and  consequently  to  the 
papillae,  show  the  flaps  in  cross  section  with  the  blood  vessel  in  the  outer 
edge  (Fig.  462,  A).  The  body  of  the  evagination  is  of  loose  connective 
tissue  and  covered  with  a  very  thin  stratified  epithelium.  In  the  base 


FIG.  462.  —  Acanthias  vulgar  is;  A,  Diagrammatic  figure  of  part  of  a  section  of  the  wall  of  the 
pregnant  oviduct  (uterus),  taken  transversely  to  the  length  of  the  tube;  bl,  blood  vessels; 
ep,  respiratory  epithelium.  Lower  layers  are  connective  tissue,  and  longitudinal  and  circu- 
lar muscle  layers  covered  by  a  layer  of  epithelium  (peritoneum).  Black  lines  in  top  of 
papilla  indicate  the  region  which  is  shown  under  greater  magnification  in  B.  B,  Small 
portion  of  the  wall  which  lies  between  the  uterine  fluid  and  the  large  blood  vessel  on  edge 
of  a  papilla;  bl,  blood  capillaries  in  outer  epithelium.  A,  X  20.  B,  X  500. 

of  each  papilla  is  another  vessel,  usually  empty  or  with  bu'  a  few  red 
blood  corpuscles  in  its  lumen.  It  is  nearly  as  large  as  the  arterial  loop 
out  in  the  edge,  and  is  the  vein  which  returns  the  blood  to  the  circulatory 
system. 

The  passage  of  the  blood  from  the  superficial  loop  of  artery  to  the 
lower  and  more  internal  vein  is  the  interesting  structural  feature  of  the 
tissue  (Fig.  462,  B).  It  does  not  flow  through  the  body  of  the  connective 
tissue,  which  is  barely  supplied  with  circulation,  but  enters  a  plexus  of 
capillaries  just  under  the  stratified  epithelium  and  so  close  to  it  that  a 
separating  blood-vessel  wall  is  hardly  discernible.  The  blood  appar- 
ently flows  in  contact  with,  or  even  within,  the  epithelium,  which  is 
weakly  stratified.  These  channels  proceed  from  the  top  of  the  papilla 


494 


HISTOLOGY 


down  to  its  base,  where  their  contents  are  collected  and  carried  away  by 
the  vein. 

Just  inside  the  vein,  and  the  connective-tissue  layer  in  which  it  lies, 
is  the  inner  circular  layer  of  smooth  muscle  which  is  found  in  all  parts 
of  the  uterine  wall.  Outside  of  this,  again,  is  the  longitudinal  smooth 
muscle  layer  which  is  covered  externally  by  a  peritoneum. 

Oxygen  certainly  passes  from  the  blood  into  the  uterine  fluid,  and  is 
taken  into  the  embryonic  blood  by  the  embryonic  gill  filaments  described 
in  Chapter  XVII.  Also,  urates  are  probably  returned  and  excreted 
through  the  parental  kidney.  Carbon  dioxide  must  also  be  returned. 
As  to  whether  any  food  materials  are  transferred  from  the  parental 


FlG.  463.  —  Myliobalis  aquila;  outline  of  a  section  through  the  wall  of  the  oviduct  in  a  plane 
corresponding  to  the  section  in  last  figure.  The  epithelium  on  the  papillas  is  respiratory. 
The  thicker  cells  lining  the  intermediate  pockets  are  food  secreting.  (After  BRINKMANN.) 


blood  to  the  embryo  through  the  uterine  fluid  is  doubtful.  The  embryo 
has  a  large  yolk  supply,  but  when  finally  born,  it  weighs  much  more  than 
the  new  egg  did.  The  oxygen  which  it  has  absorbed  might  account  for 
part  of  this  extra  weight,  also  the  water.  On  the  other  hand,  the  carbon 
dioxide  and  urea  thrown  off  would  detract  from  this  balance.  It  thus 
remains  an  open  question  as  to  whether  the  parent  furnishes  its  young 
with  food  other  than  oxygen  and  its  original  supply  of  yolk. 

We  are  so  fortunate  in  this  case  of  doubt  as  to  have  evidence  on  which 
to  base  a  decision.  There  are,  among  the  species  of  elasmobranch  fishes, 
a  perfect  series  of  stages,  beginning  with  those  that  lay  an  egg,  passing 
through  many  sharks  and  rays  which  hold  the  young  in  a  uterus  to 
develop  on  the  yolk,  as  above  in  the  dogfish,  and  gradually  terminating 


NOURISHING  MEMBRANES 


495 


-ft*. 


with  members  of  the  group  that  very  clearly  nourish  the  young  with  a 
fluid  food  furnished  by  the  walls  of  the  uterus. 

We  shall  study,  as  an  example  of  this  latter  histological  condition, 
the  uterine  wall  of  the  elasmobranch  fish  Myliobatis  aquila,  a  ray  found 
in  the  Mediterranean  Sea  and  stud- 
ied by  Brinkmann.  In  the  uterus 
of  this  fish  the  lining  epithelium 
is  simple,  and  is  arranged  on  a 
series  of  papillae  to  present  a  large 
underlying  blood  supply  to  the 
surface.  This  is  just  as  it  was 
arranged  in  Acanthias. 

In  addition  to  this  arrangement, 
however,  such  of  the  epithelium  as 
lies  in  the  spaces  between  the 
papillae  is  specialized  so  as  to  be 
able  to  take  materials  from  the 
blood,  and,  after  having  elabo- 
rated them  into  a  food  material 
that  is  particularly  adapted  to 
the  needs  of  the  embryo,  to  dis- 
charge them  into  the  uterine  fluid 
in  which  the  embryo  lies,  where 
they  will  be  accessible  for  its  nour- 
ishment. 

This  food  material  is  discernible 
in  the  uterine  fluid  as  a  milky  con- 
stituent, and  while  its  chemistry  and  the  physiology  of  its  assimilation 
by  the  embryo  have  been  studied,  there  remains  much  interesting  work 
to  be  done.  Figure  463  shows  a  low  power,  outline  view  of  a  vertical 
section  of  this  uterine  lining  to  show  the  relations  between  the  respiratory 
and  nourishing  epithelium;  while  in  Figure  464  is  shown  a  much  enlarged 
view  of  a  section  of  one  of  the  "pockets"  of  nourishing  epithelium.  It 
can  be  seen  here  that  cellular  elements  (leucocytes),  as  well  as  the  se- 
creted materials,  are  passing  through  or  between  the  epithelial  cells. 
The  secreted  substance  is  observable  as  a  series  of  vacuoles  which  are 
slightly  larger  in  the  distal  cytoplasm  than  in  the  proximal.  The  writers 
believe,  from  study  of  Brinkmann's  figure  and  without  having  seen 
actual  sections  of  this  material,  that  the  two  kinds  of  epithelial  nuclei 
described  by  this  author  are  the  same,  of  which  the  lighter  ones  are  such 
as  have  been  cut  by  the  knife  in  sectioning,  and  thus  have  stained  more 
slightly. 

We  shall  next  study  the  extreme  of   this  condition,  as  found  in  a 


FlG.  464.  —  Myliobatis  aquila;  a  higher  mag- 
nification of  one  of  the  nutritive  pockets 
shown  in  the  last  figure,  sec.g.,  cytoplasmic 
secretion  granules;  lu.,  leucocytes.  (After 
BRINKMANN.) 


496 


HISTOLOGY 


mammal,  where  the  young  start  off  with  practically  no  yolk  supply  and 
depend  entirely  upon  materials  from  the  parent's  blood  for  everything. 
We  shall  describe  briefly  this  uterine  surface  together  with  the  embryonic 
membranes  by  which  the  young  animal  is  able  to  take  advantage  of  these 
supplies. 

The  normal  adult  uterus,  in  many  mammals,  consists  of  an  outer 
muscular  layer  with  an  inner  mucous  layer  that  is  composed  of  a  thick 
subepithelial  connective  tissue  containing  many  lymph  spaces  and 
blood  vessels.  The  epithelium  which  lines  the  mucous  layer  is  a  simple 


FIG.  465.  —  Part  of  a  longitudinal  section  through  the  uterine  wall  of  a  cat.  The  foetal  mem- 
branes of  a  very  young  embryo  are  seen  in  situ.,  applied  to  it.  am.,  amnion;  ch.,  chorion; 
bl.,  blood.  Dotted  lines  show  region  from  which  next  section  was  drawn,  x  45. 


columnar  epithelium  which  is  invaginated  into  a  great  many  simple,  or 
sometimes  branched,  tubular  glands  that  reach  to  the  muscle  layer  and 
secrete  no  special  fluid.  The  outer  part  of  this  layer  with  its  epithelium 
and  part  of  the  glands  are  broken  up  and  thrown  off  at  a  period  called 
the  menstrual  period.  This  lasts  a  few  days,  after  which  the  lost  epi- 
thelium is  regenerated  from  the  remaining  portions  of  the  glands. 

The  fcetal  membrane  is  composed  of  two  layers,  an  outer  layer  cov- 
ered externally  with  a  simple  epithelium,  under  which  is  some  embryonic 
mesodermal  tissue.  This  is  the  chorion.  Also  an  inner  layer  composed 
likewise  of  a  simple,  but  inner,  epithelium  resting  on  a  mesodermal 
layer.  This  is  called  the  amnion.  These  two  layers  are  joined  together 
by  their  mesodermal  surfaces,  the  line  of  contact  being  marked  by  a  very 


NOURISHING  MEMBRANES 


497 


>,. 


loose  connective  tissue.  They  thus  form  what  is  practically  a  single 
mesodermal  layer  lined  on  its  outer  and  inner  surfaces  with  a  simple  epi- 
thelium. They  are 

shown    in    semi-dia-  #&•  aw>: 

gram  in  Figure  465. 

This  membrane  is  P*^^^^%,  Jif**.** 
a  part  of  the  em- 
bryo's body  at  this 
time,  and  the  fcetal 
circulation  extends 
extensively  into  it  as 
a  plexus  of  small 
vessels  in  the  meso- 
dermal core.  When 
the  embryo  attains 
a  certain  size,  this 
membrane  is  applied 
by  its  chor ionic  sur- 
face to  the  internal 
epithelial  surface  of 
the  uterus  and  forms 
some  sort  of  adhesion 
to  it.  Figure  465 
shows  a  semi-dia- 
grammatic represen- 
tation of  this  in  the 
uterus  of  a  cat,  while 
Figure  466  shows  an 
enlarged  view  of  part 
of  this  same  section. 
This  relation  contin- 
ues to  be  maintained 

-     ,         FIG.  466.  —  Enlargement  of   part  of  Figure  465.     ep.am.,  epi- 
in   Some    parts  OI    tne        dermal  layer  of  amnion;  mes. am.,  mesodermal  layer  of  amnion ; 


ep'ch. 


conn.  t. 


mes.ch.,  mesodermal  layer  of  chorion;  ep.ch.,  epithelial  layer  of 
chorion  merged  with  the  simple  epithelium  that  lines  the  uterus ; 
conn.t.,  connective  tissue;  bl.t  blood  cells  of  embryo.  X  520. 


contact.  But  in 
others  a  more  inti- 
mate association  is 
formed.  This  occurs  in  different  regions  in  different  mammals.  In 
man  it  is  on  one  side  of  the  uterus,  and  it  is  developed  as  follows  (Fig. 
467). 

The  epithelium  layer  of  the  chorion  is  evaginated  into  a  series  of 
branching  villi,  which  push  down  (outward  or  distally  with  regard  to 
the  chorion,  proximally  with  regard  to  the  uterine  epithelium)  into  the 
mucous  layer  of  the  uterus.  As  the  villi  advance,  the  surface  of  the 

2K 


498 


HISTOLOGY 


uterine  layer  degenerates,  or  it  may  already  be  broken  up  by  the  men- 
strual process.     It  is  destroyed  and  removed  until  only  the  lower  parts 

of  the  gland  are  left,  to- 


gether with  a  residual  con- 
nective-tissue layer  about 
one  third  as  thick  as  the 
original  membrane. 

The  distal  ends  of  the 
uterine  blood  vessels  are 
also  lost  in  this  process, 
and  the  maternal  blood 
comes  out  of  the  free  ends 
of  the  arteries  and  circu- 
lates in  the  open  spaces 
that  lie  between  the  uterus 
and  chorion,  and  among 
the  villi.  As  the  broken 
ends  of  the  uterine  veins 
also  remain  open,  this  blood 

FIG.  467.  —  Diagram  of  the  relations  of  the  foetal  mem-  is  returned  through  them 
branes  to  the  uterus  in  man  at  the  close  of  pregnancy.  to  fa  matemal  circula- 
( After  SCHAPER.)  •  .... 

tion.  Figure  467  is  a  dia- 
gram after  Schaper  to  illustrate  this  condition. 

During  this  development  the  simple  epithelium  on  the  villi  has  pro- 
liferated an  outer  layer,  which  differs  in  appearance  from  the  original 
layer,  which  now  lies  in  a  basal  portion  on  the  connective  tissue. 
This  second  and  distal  layer  is  syncytial,  in  that  its  cell  boundaries  are  not 
demonstrable.  It  is  thickest  on  the  tips  of  the  villi,  and  is  incomplete 
nearer  the  chorion.  Where  it  is  thickest,  it  is  developed  into  tuberosi- 
ties  much  like  those  on  the  umbilical  cord.  When  this  membrane  is 
torn  away  from  the  uterine  wall,  the  blood  vessels  close  and  establish  new 
connections  between  arteries  and  veins,  the  reticular  tissue  is  thick- 
ened, and  a  new  covering  of  epithelium  is  regenerated  from  the  re- 
maining portions  of  the  glands. 

It  should  be  noticed,  in  the  apparatus  described  above,  that  the  food 
materials  and  other  materials  are  exchanged  directly  from  parental 
blood  to  embryonic  blood,  and  vice  versa  through  only  a  few  thin  cells. 
In  the  elasmobranch  fishes,  on  the  other  hand,  a  uterine  fluid  stood  as 
an  additional  agent  of  transfer.  Also  some  or  all  of  the  food  material 
probably  entered  the  young  fish's  body  through  the  digestive  tract  in- 
stead of  directly  into  the  blood. 

The  work  of  nourishing  the  offspring  from  the  parental  body  is  not 
finished  even  with  their  birth,  in  the  mammals.  It  is  then  taken  up  by 


NOURISHING  MEMBRANES 


499 


the  mammary  glands  from 
which  the  young  suck  an 
epithelial,  glandular  se- 
cretion called  the  milk. 
These  glands  consist  of 
an  embryonic  invagina- 
tion  of  the  stratified  epi- 
thelium into  a  series  of 
aveolo-tubular  glands 
(Fig.  468).  In  the  acini 
and  ducts  of  these  invagi- 
nations,  only  the  basal 
layer  of  cells  persists  as 
a  simple  epithelium  which 
is  the  secreting  layer  of 

the   gland.  FIG.  468.  —  An  acinis  from  a  functional  mammary  gland  of 

The  mode  of   Secretion       the  cat-    The  lumen  is  filled  with  the  watery  secretion. 
, .  ,  Several  of  the  cells  are  secreting  large  fat  droplets  which 

IS  peculiar,  when  we  COn-      are  stained  black  with  the  osmic  acid,     x  1300. 

sider  the  fact  that  other 

oils  (sebaceous  and  odorous)  are  usually  produced  by  a  degeneration 


FIG.  469.  —  Part  of  the  foetal  membranes  of  a  tern,  Larix.  ent.,  entoderm  whose  cells  are  engaged 
in  securing  nourishment  from  the  yolk.     X  435. 

and  disquamination  of  the  cell.     In  this  form  of  tissue  the  secretion  is 
formed  in  the  distal  portion  of  the  rather  short  cells,  and  is  carried  to 


5oo 


HISTOLOGY 


the  surface  and  discharged  as  a  droplet,  the  vacuole  from  which  it  came 
being  closed  behind  it. 

These  glands  are  probably  specializations  or  phylogenetic  derivatives 
of  sweat  glands  or  of  primitive  glands  from  which  the  sweat  glands  were 
also  derived.  This  is  shown  by  their  simple  epithelial  lining  and  the 
manner  in  which  they  produce  both  droplets  of  fat  and  a  supply  of  the 
watery  constituents  of  the  milk. 

The  first  part  of  this  secretion,  just  after  childbirth,  is  handled  by 
certain  lymph  cells,  or  amoebocytes,  which  crawl  between  the  cells  and  lie 
in  the  lumen.  Here  they  receive  the  secretion  and  carry  it  out.  They 
are  called  the  colostrum  corpuscles. 

We  must  also  consider  here,  certain  embryonic  membranes  which 
are  used  to  take  food  material  from  a  yolk  or  store  of  food  instead  of 
from  a  maternal  membrane.  The  parent  is  not  concerned  in  this  pro- 
cess other  than  by  the  fact  that  she  previously  furnished  the  yolk  and  that 
the  process  may  take  place  in  her  body  as  well  as  outside  of  it.  Such 
cases  are  well,  but  narrowly,  represented  by  the  yolk  membranes  of  a 

bird,  the  tern,  and 
those  of  a  fish,  the 
toadfish. 

In  the  tern,  the 
development  of  the 
embryo  results  in  a 
four-layered  mem- 
brane which  stretches 
from  the  body  of  the 
young  bird  over  the 
yolk  (Fig.  469).  The 
two  inner  layers  of 
the  four  develop  a 
system  of  capillary 
circulation.  This 
plexus  appears  de 
noi>o,  and  the  blood 
corpuscles  appear  as 
a  part  of  the  tissue 
that  lie  ( ?)  within  its 
walls.  The  outer 
layer  is  a  simple  epi- 
thelium derived  from 
the  ectoderm,  and  it  has  temporarily  assumed  the  function  of  a  respira- 
tory membrane. 

The  cells  of  the  inner  layer  are  very  much  specialized  and  are  used 


FIG.  470.  —  Section  through  an  embryo  of  the  toadfish,  Opsanus. 
pb.n.,  nuclei  of  the  periblastic  syncytium  which  elaborates  yolk 
for  the  use  of  the  young  fish,  x  250. 


NOURISHING  MEMBRANES  501 

to  take  yolk,  and,  having  properly  prepared  it,  to  transfer  it  as  a  solution 
into  the  blood.  The  cells  are  very  large,  and  the  nucleus  lies  in  the  prox- 
imal end.  The  distal  end  shows  masses  of  yolk  which  are  in  process  of 
food  elaboration. 

The  early  stages  of  a  teleost  fish  embryo  show  a  proximal  layer  of 
tissue  which  lies  on  the  yolk  and  is  probably  used  at  this  early  period 
to  secure  the  nourishment  from  it.  The  layer  is  evidently  cytoplasmic, 
but  it  shows  no  cell  boundaries,  and  is  therefore  syncytial  (Fig.  470). 
It  contains  enormous  nuclei  of  a  peculiar  shape  and  chromatin  pattern. 
These  nuclei  are  known  as  the  paraUastic  nuclei,  and  they  multiply  by 
amitosis.  Later  they  are  done  away  with,  and  another  form  of  nutritive 
tissue  is  substituted. 

Technic. — The  nourishing  tissues  are  easily  cut  in  paraffin  and  should 
be  studied  by  individual  sections  fixed  and  treated  for  the  best  cytological 
results.  In  some  few  cases  it  is  desirable  to  understand  the  larger  his- 
tological  relations,  and  for  this  purpose  bulk-stained  material  should 
be  cut  in  celloidin.  Sometimes  serial  sections  of  particular  regions  are 
necessary,  and  bulk-stained  material,  cut  in  medium  or  soft  paraffin,  will 
give  the  best  results  here. 


LITERATURE 

BRINKMANN.  "Histologie,  Histogenese,  und  Bedutung  der  Mucosa  uteri  einiger  vivi- 
paren  Haie  und  Rochen,"  Mitt.  Zool.  Stat.  zu  Neapel,  Band  XVI,  1903. 

BROUHA.  "Les  phenomenes  histologique  de  la  Secretion  lactee,"  Anat.  Am.,  Band 
XXVII,  p.  464  (and  later  in  Arch,  de  Biol.). 

STRAHL  UND  HAPPE.  "Neue  Beitrage  zur  Kenntnis  von  Affenplacenten,"  Anat.  Am., 
Vol.  XXIV,  1904,  S.  454- 

WILSON,  H.  V.  "The  Embryology  of  the  Sea-bass,  Serranus"  Bull,  of  the  U.  S.  F.  C., 
1893. 


CHAPTER   XXIV 
TECHNIC 

THE  technic  of  cytological  and  histological  research  has  assumed 
formidable  and  intricate  proportions  of  detail.  Its  fundamental  ideas, 
however,  remain  the  same  and  probably  95  per  cent  of  the  de- 
tailed modern  methods  are  based  upon,  or  elaborated  from,  these  first 
principles. 

The  student  is  advised  to  read  and  digest  the  following  general  outline 
of  the  principles  of  technic,  and  then  execute  the  several  complete  sched- 
ules. After  this,  if  he  wishes  to  further  master  technic,  he  should  pre- 
pare for  study  the  specimens  which  appear  in  each  chapter,  following 
the  outline  directions  given  in  special  cases,  and  these  will  afford  him 
a  sufficiently  varied  and  extensive  practice  for  all  purposes.  The  "Micro- 
scopist's  Vade  Mecum,"  by  Lee,  should  be  at  hand  and  referred  to  in 
this  connection. 

The  methods  are  in  all  cases  the  best  that  the  writers  have  actually 
had  experience  with.  They  are  not  picked  out  with  reference  to  an 
inexperienced  student  and  a  poorly  equipped  laboratory,  but  call  for  the 
best  of  instruments  and  reagents,  and  an  experienced  instructor.  In 
different  hands,  other  methods  will  sometimes  be  found  to  do  the  work 
better  in  certain  cases,  and  the  examples  found  below  are  offered  as  a 
convenience  or  a  starting-point. 

GENERAL  OUTLINE 

Most  tissues  cannot  be  studied  in  a  fresh  condition  because  of  several 
obstacles ;  they  are  too  thick  and  cannot  be  cut  into  thin  slices  on  account 
of  their  texture.  Besides,  their  parts  are  nearly  of  one  common  color 
and  refractive  index,  which  makes  the  structure  indistinct  at  the  best, 
and,  lastly,  they  will  soon  decay  or  dry  and  are  not  permanent. 

So  our  technic  is  devised  to  get  slices  thin  enough  to  see,  to  change 
the  color  and  refractive  index  to  more  favorable  conditions,  and  to  make 
more  or  less  permanent  preparations. 

The  majority  of  tissues  containing  much  protoplasm  must  first  be 
fixed,  which  means  killed  by  some  medium  that  leaves  them  in  a  con- 

502 


TECHNIC  503 

dition  that  is  as  near  as  possible  to  life,  and  that  will  withstand  change 
for  some  little  time.  Fixation  results  in  a  whitened  appearance  and  a 
firmness,  or  even  a  considerable  hardening,  of  the  tissue.  The  fixatives 
used  are  usually  mineral  acids  and  salts,  organic  acids,  and  sometimes 
heat.  A  natural  death  of  the  tissue  would  result  in  its  speedy  disorgan- 
ization. 

The  second  step  consists  of  the  use  of  fluids  that  will  further  harden 
and  preserve  the  tissue,  and  at  the  same  time  will  prepare  the  way  for 
subsequent  treatment  by  removing  all  unnecessary  or  injurious  sub- 
stances. The  fixatives,  especially,  must  be  removed  to  avoid  artifacts, 
due  to  crystallization,  etc.  Alcohol  is  a  fluid  admirably  adapted  to  fur- 
nish these  two  results,  especially  when  water  is  first  used,  in  some  cases, 
to  remove  substances  insoluble  in  alcohol.  Lastly,  the  tissue  must  be 
dehydrated  for  purposes  we  shall  soon  see,  and  here  again  alcohol  is  the 
best  reagent. 

This  decides  the  use  of  alcohol  as  fulfilling  all  purposes,  and  it  is 
used  in  gradually  increasing  strengths  to  prevent  too  rapid  osmosis 
and  consequent  distortion.  Sometimes  additional  substances  are  used 
in  the  alcohol  to  help  remove  the  fixative,  as  iodide  of  potassium  after 
mercury  bichloride. 

The  third  step  is  decided  by  our  ultimate  object  of  getting  the  piece 
of  tissue  embedded  in  paraffin  (or  in  celloidin)  so  that  it  may  be  cut  in 
thin  sections.  Paraffin  will  not  mix  with  alcohol,  so  an  intermediate 
fluid,  some  oil  that  will  mix  with  both  alcohol  and  with  melted  paraffin, 
is  employed  and  substituted  for  the  alcohol.  This  step  is  known  as 
clearing.  Common  clearing  reagents  are  cedar  oil,  toluol,  xylol,  etc. 
Chloroform  is  also  a  valuable  clearing  reagent. 

No  obstacle  now  exists  to  prevent  melted  paraffin  from  being  sub- 
stituted for  the  clearing  reagent.  This  is  done  in  an  oven  just  warm 
enough  to  keep  the  paraffin  melted,  and,  when  infiltrated,  the  mass  should 
be  poured  out  into  a  paper  box,  the  bit  of  tissue  oriented,  and  the  mass 
cooled.  When  the  block  is  cooled,  we  find  that  the  tissue  is  completely 
embedded  in,  and  infiltrated  by,  the  paraffin.  The  mass,  which  is  hard, 
can  be  cut  with  a  knife  into  very  thin  sections  without  injuring  the 
tissue,  or  altering  its  structure  or  the  relative  position  of  its  parts. 

The  sections  should  now  be  caused  to  adhere  to  a  glass  microscope 
slide,  the  paraffin  melted  off  with  xylol  or  other  clearing  reagent,  and 
the  specimen  then  freed  from  the  clearing  reagent  by  alcohol  and  from 
alcohol  by  water.  The  proper  dyes  will  now  stain  the  cell  and  its  organs 
with  a  differential  stain  that  should  enable  all  parts  to  be  seen.  When 
stained,  the  slide  must  be  dehydrated  again,  then  cleared,  and  lastly 
a  drop  of  balsam  placed  on  the  section,  which  is  then  covered  with  a  thin 
cover  glass. 


504 


HISTOLOGY 


This  is  the  best  way  known  in  most  cases  of  seeing  an  accurate  picture 
of  the  details  of  structure  in  a  tissue.  Good  results  can  be  attained  by 
staining  the  tissue  after  fixation,  and  before  it  is  dehydrated.  In  that 
case,  when  the  sections  are  cut  and  mounted  on  the  slide,  they  may  be  at 
once  freed  from  paraffin  and  mounted  in  the  balsam.  This  way  of 
staining  in  toto  or  in  bulk,  although  shorter,  seldom  shows  the  detail  and 
accuracy  of  sections  stained  on  the  slide.  It  is  a  valuable  method, never- 
theless. 

In  case  embedding  in  celloidin  is  decided  upon,  the  material  is  dehy- 
drated and  immersed  in  a  solution  of  celloidin  or  photoxylin  dissolved  in 
ether  or  absolute  alcohol,  or,  as  is  more  commonly  done,  in  a  mixture 
of  both.  After  a  long  soaking  to  allow  of  a  good  infiltration,  the  solution 
is  thickened  by  evaporation  or  the  addition  of  a  stronger  solution,  when 
the  mass,  now  nearly  solid,  may  be  hardened  in  chloroform  or  its  vapor, 
and  then  cut  into  sections  which  are  not  as  thin  as  paraffin  sections,  but 
usually  have  a  peculiar  beauty  of  their  own,  due  to  their  not  having  been 
subjected  to  the  shrinking  action  of  cooling  paraffin.  As  in  the  paraf- 
fin method,  these  sections  can  be  stained  separately  before  mounting,  or 
the  material  can  be  previously  stained  in  bulk,  which  is  perhaps  better 
for  the  celloidin  method,  and  permits  of  immediate  mounting  when  cut. 
Otherwise  the  sections  are  stained  as  "free  sections"  or,  in  rare  cases, 
on  the  slide. 

The  student  should  consult  Lee  for  the  cutting  of  sections  of  bone, 
etc.,  and  other  exceptional  cases. 


FIRST  EXAMPLE 
Secure  a  salamander  (Necturus  preferred). 


A.   Cut  portions  of  liver  into  cubes  of  10  to  15  mm. 
or  less. 

Large  enough  for  a  view 
of  its  structure.  Small 
enough  for  penetration  of 
fixative  and  other  processes. 

Place  pieces  in  a  saturated  solution  of  corrosive 
sublimate,  in  water,  to  which  5  per  cent  of  glacial 
acetic  acid  has  been  added. 
Allow  to  remain  one  hour  to  two  hours. 

To  fix. 

B.   Rinse  and  move  to  70  per  cent  alcohol  for  one 
hour.     Change   the  alcohol  once.     Place  in  25  cc.  or 
50  cc.  of  80  per  cent  alcohol  for  one  or  two  hours  more. 

To  harden,  and  to  remove 
such  sublimate  and  mer- 
cury compounds  as  are  sol- 
uble in  water  and  alcohol. 

TECHNIC 


505 


Add  i  cc.  of  solution  of  iodine  (\  per  cent)  and 
potassium  iodide  (i  per  cent)  in  water.  Soak  in  this 
for  three  hours  or  more.  Place  in  clean  alcohol  (80 
per  cent)  for  one  or  more  hours.  Repeat  the  treat- 
ment with  potassium  iodide  and  then  remove  it  by 
soaking  it  again  in  80  per  cent  alcohol. 

To  prevent  the  crystalliza- 
tion of  the  sublimate  and  to 
remove  certain  compounds 
of  mercury  that  would  other- 
wise spoil  the  specimen. 

C.  Place  in  95  per  cent  alcohol  for  one  hour,  drain 
on  blotting  paper,  and  place  in  absolute  (99  per  cent) 
alcohol  for  one  or  more  hours.  Change  once. 

To  dehydrate. 

D.   Place  in  a  small  quantity  of  xylol  or  toluol.     If 
a  milky  color  appears  and  persists,  dehydration  is  not 
complete  and  the  specimen  should  be  returned  to  fresh 
absolute  alcohol  for  one  half  hour  and  then  back  to 
xylol. 
The  specimen  will  now  become  translucent  or  even 
transparent  and  is  then  ready  for  embedding. 

To  replace  the  alcohol 
with  a  substance  that  will 
mix  with  melted  paraffin. 

E.  Place  in  melted  paraffin  of  52-56  melting  point 
in  an  open  dish. 

To  infiltrate  with,  and 
embed  the  specimen  in  par- 
affin. 

Keep  in  a  water  bath  which  is  slightly  warmer  than 
the  melting  point  of  the  paraffin.  Change  the  paraffin 
once  or  twice,  and  in  from  one  to  two  hours  the  speci- 
men will  be  properly  infiltrated  and  ready  to  embed. 

F.  Embed  in  a  paper  box.  See  that  marks  on  the 
box  show  in  which  plane  the  sections  are  to  be  cut.  It 
is  well  to  adopt  some  regular  position  and  try  to  em- 

Instructor  must  supervise 
this  work. 

bed  so  that  the  sections  should  always  be  cut  from  one  surface  or  side  of  the 
box,  preferably  the  under  surface.  Cut  sections,  on  a  rotary  microtome,  5  to  10 
microns  thick.  Attach  to  a  clean  slide  by  placing  several  drops  of  water  on  the 
slide  and  floating  the  sections  on,  taking  care  to  remove  all  air  bubbles  caught  under 
the  folds  of  the  sections.  Heat  on  the  top  of  water  bath  to  a  point  not  over  15° 
to  10°  below  the  melting  point  of  the  paraffin  used.  If  the  section  melts  at  any  point, 
such  part  will  not  afterwards  adhere.  When  the  section  has  straightened  out  under 
this  gentle  heat,  allow  the  water  to  drain  from  under  it  and  arrange  it  to  suit  with  a 
needle  or  scalpel;  never  work  with  one  tool,  but  have  one  pointed  instrument  in  each 
hand  so  that  if  the  section  sticks  to  one  instrument  the  other  can  be  used  to  release  it. 
Surface  tension  will  make  or  mar  the  arrangement  according  as  it  is  understood  and 
used,  or  ignored  and  fought  with.  Thus  two  rows  of  flattened  sections,  freely  floating, 
may  absolutely  refuse  to  be  drawn  together  to  make  room  for  a  third  row  outside  of 
them.  If  now  the  point  of  a  scalpel  be  drawn  between  them  several  times  to  break  up 
the  invisible  film  of  paraffin  on  the  water,  they  will  come  together  themselves.  In  fact, 
they  could  not  be  kept  apart. 

When  arranged,  place  the  slides  in  a  dry,  well-ventilated  place  for  six  or  more 
hours  to  dry  out  completely.  The  sections  will  then  adhere  through  any  other  pro- 
cess. When  the  fixative  contains  chromic  acid  or  any  of  its  compounds,  egg  fixative 
must  be  very  slightly  smeared  on  the  slide  before  placing  the  water  and  sections 


5o6 


HISTOLOGY 


upon  it.     It  is  perhaps  best  to  make  a  practice  of  using  the  egg  fixative  in  all 
cases. 

The  slides  can  now  be  stored  for  some  time  in  dust-proof  boxes.  It  is  best,  how- 
ever, to  proceed  soon  after  this  to  stain  and  mount  them.  We  shall  continue  our  de- 
scription of  a  particular  example. 


G.   Place  the  slide  in  xylol  for  three  minutes. 

To  remove  the  paraffin. 

Rinse  it  in  absolute  alcohol  and  then  in  95  pei  cent 
alcohol.  It  is  now  ready  to  stain. 

To  remove  the  xylol. 

H  .  Place  the  slide  in  a  i  per  cent  solution  of  iron 
alum. 

To  mordaunt  it  or  pre- 
pare it  for  the  stain. 

Rinse  in  distilled  water  and  transfer  to  a  one  fourth  of  i  per  cent  solution  of  haema- 
toxylin  in  distilled  water.  Experience  will  tell  how  long  to  stain.  From  six  to  twelve 
hours  is  usually  right.  The  longer  stain  will  bring  out  the  achromatic  and  cytoplasmic 
structures  best. 

When  stained,  the  slides  must  be  rinsed  and  returned  to  a  somewhat  weaker  solu- 
tion of  the  same  iron  alum  that  was  used  to  mordaunt  them.  They  must  be  watched 
while  the  color  is  extracted  and  had  best  be  frequently  taken  out,  rinsed,  and  examined 
with  an  old  microscope  to  see  that  the  proper  reaction  has  taken  place.  They  will 
look  darker  when  finally  mounted  than  when  thus  examined  in  water.  The  stained 
slides  should  now  be  washed  in  gently  running  tap-water  or  in  a  number  of  changes  of 
fresh  water  to  remove  all  traces  of  the  iron  alum.  They  are  then  ready  for  — 

MOUNTING 


7.  Place  the  slides  in  95  per  cent  alcohol  for  a  few 
minutes.  Transfer  them  to  absolute  alcohol  for  about 
five  minutes. 

To  remove  all  water. 

Place  them  in  two  successive  baths  of  xylol. 

To  remove  all  alcohol  and 
replace  it  with  an  oil  that 
will  mix  with  balsam. 

Drain  off  excess  of  xylol  and  promptly  place  a  drop  of  some  good,  acid-free  balsam 
solution  on  the  specimen,  which  should  still  be  wet  with  xylol.  It  must  at  no  time  be 
allowed  to  dry.  Use  Griibler's  damar  balsam  dissolved  in  xylol.  The  cover  glass,  of 
number  one  glass,  should  now  be  placed  in  position  without  pressing  on  its  top  when  in 
place,  and  the  slide  laid  flat  in  a  warm  place.  Any  air  bubbles  which  happen  to  be 
inclosed  will  find  their  own  way  out  in  a  few  days,  if  the  proper  amount  of  balsam  is 
present.  Balsam  may  be  added  in  small  drops  at  the  edge  of  the  cover.  The  balsam 
should  be  diluted,  if  too  thick,  with  pure,  fresh  xylol. 

The  above  is  a  single  concrete  example.'  We  shall  suggest  as  a  desirable  variation 
the  following  — 

SECOND  EXAMPLE 
AA.  Fix  bits  of  the  same  tissue  in  Flemming's  solution  of  — 

1  per  cent  chromic  acid        ........          15  parts 

2  per  cent  osmic  acid  .........  4  parts 

Glacial  acetic  acid       .  i  part 


TECHNIC  507 

Use  smaller  or  thinner  portions  of  tissue  if  possible  and  place  in  a  rather  small  quan- 
tity of  the  fluid.  Do  not  change  the  fluid.  Fix  for  twenty-four  hours  to  ten  days. 
BB.  Same  as  (B),  except  that  the  tissue  should  be  washed  for  a  time  in  clean  tap 
or  distilled  water  and  the  iodide  treatment  dispensed  with.  Subsequent  steps  are 
the  same. 

THIRD  EXAMPLE 

Another  variation  that  should  be  tried  is  as  follows:  Treat  tissue  according  to 
(A)  and  (B). 

HH.  Stain  for  twenty-four  hours  in  a  solution  of  borax  carmine.  Decolorize  for 
twelve  hours,  with  frequent  changes,  in  70  per  cent  alcohol  to  which  hydrochloric  is 
added  (6  drops  to  each  100  cc.). 

Then  proceed  as  in  (C),  (D),  (£),  (F),  and  (G),  except  that  the  alcohols  should  be 
omitted  in  this  latter  step.  As  the  sections  are  already  stained,  they  may  then  be 
mounted  as  in  (/),  except  that  the  first  treatment  (with  alcohol)  may  be  omitted  as  the 
sections  are  already  free  of  water. 

FOURTH  EXAMPLE 

Celloidin  embedding  should  now  be  practiced  as  follows.  Proceed,  with  the  same 
tissue,  as  in  (A),  (B),  (HH),  and  (Q. 

/.  Place  in  a  quantity  of  celloidin  dissolved  in  equal  parts  of  ether  and  absolute 
alcohol.  The  amount  of  celloidin  should  be  from  i  to  2  per  cent.  This  should  be  in 
a  well-stopped  bottle,  and  a  long  treatment  of  days  or  even  weeks  is  beneficial.  Forty- 
eight  hours  will  do.  The  strength  of  the  celloidin  solution  should  be  increased  to 
6  per  cent,  and  finally  the  object  should  be  placed  on  the  end  of  a  cork  surrounded  with 
a  covering  of  thick  celloidin  solution,  and  as  it  becomes  firm  on  the  surface,  plunged 
into  pure  chloroform  (on  the  carrier)  for  an  hour  or  more,  to  harden. 

K.  When  hard,  it  should  remain  in  a  mixture  of  cedar  oil  (or  cedarwood  oil) 
and  chloroform,  equal  parts,  for  another  hour,  when  the  whitened  celloidin  will  become 
clear.  Sections  may  now  be  cut  with  a  knife  wet  in  the  chloroform-cedar  oil  mixture. 
A  soft  brush  must  be  used  to  keep  the  knife  flooded,  and  chloroform  must  often  be 
added  to  compensate  for  evaporation.  When  the  cedar  oil  is  in  excess,  the  celloidin 
softens  or  even  melts. 

L.  The  free  sections  may  be  floated  on  a  slide  with  a  brush,  drained,  and  at 
once  mounted  in  balsam.  They  are  already  dehydrated  and  cleared  and  need 
only  the  balsam  and  a  cover  glass. 

One  more  method  should  be  carried  out  in  the  concrete  as  follows  in  this  — 

FIFTH  EXAMPLE 

Prepare  a  bit  of  rat  testis  by  fixation,  etc.,  as  in  (A),  (B),  (HH},  and  then  restain 
it  in  Mayer's  haemalum  (see  Lee)  for  twelve  to  eighteen  hours.  Extract  the  stain  for 
six  to  ten  hours  in  i  per  cent  alum  water  (common  alum).  Dehydrate  (C),  infil- 
trate with  celloidin  (/),  and  clear  as  in  (K},  except  that  the  specimen  is  free  and  not 
placed  on  a  cork  or  other  carrier. 

Now  embed  (F)  and  cut  thin  sections ;  these  may  be  either  handled  free  (L)  by 
dissolving  the  paraffin  or  floated  with  water  on  the  slide  (G)  and  mounted,  or 
even  fastened  to  the  slide  and  restained  before  mounting  if  the  stain  has  proved  unsat- 
isfactory. 

A  favorite  method  in  medical  work,  where  fairly  thick  sections  from  the  tissues  of 
mammals  are  desired,  is  to  infiltrate  unstained  material  with  celloidin,  harden  in 


508  HISTOLOGY 

moderately  strong  alcohol  (80  per  cent),  cut,  stain  with  haematoxylin  and  eosin,  and 
then  clear  and  mount. 

The  preceding  methods  are  sufficient  for  most  work.  Some  of  the  steps,  asfixa- 
tion  and  staining,  have  hundreds  of  variations.  A  few  of  these  variations  have  been 
mentioned  after  the  different  chapters  in  regard  to  some  of  the  more  difficult  tissues. 
For  methylene  blue  staining  intra  vitem,  and  the  various  silver  and  gold  methods  and 
others,  see  Lee. 


INDEX 


Abraliopsis,  light  tissue,  128. 

Acanthias,     early     reproductive     cells,     421; 

teeth,  291-294;   uterus,  493. 
Accessory  chromosome,  442;    relation  to  sex, 

450-45 i- 

Accessory  nucleus,  428. 

Achirus,  nerve  cell,  186. 

Achromatic  figure,  of  mitosis,  25. 

Acid  cells,  of  enteron,  300. 

Acidophile  cells,  313. 

Acinus,  53. 

Acrosome,  of  spermatozoon,  427. 

Adhesive  tissues,  409;  of  Berae,  410;  of 
cephalopods,  414;  of  ccelenterates,  410; 
of  insects  (beetle),  415;  of  leech,  413;  of 
Mollusca,  411;  of  mussel,  411;  of  Proto- 
zoa, 409;  of  Remora,  414;  of  worm,  413. 

Adventitia,  160. 

^Enigma,  byssus,  412. 

jEsthenosoma,  spine,  378. 

Afferent  process,  of  nerve  cell,  176. 

Alimentary  structures,  of  Paramcecium,  284. 

Alimentary  tissues,  general,  279-280;  for  ab- 
sorption, 282;  of  Bdellura,  286;  classifica- 
tion of,  279-281;  for  conduction,  282;  for 
digestion,  283;  gastric,  283;  of  Hydra, 
286;  for  lubrication,  281;  for  mastica- 
tion, 281;  pancreatic,  283;  serous,  284; 
of  sponges,  285. 

Alligator,  lubrication  of  eye,  397;  mucous 
cell,  392. 

Allolobophora,  blood  vessels,  153. 

Ameiurus,  egg  follicle,  456;   static  tissue,  213. 

Amitosis,  37;  epithelium  of  Guinea  pig,  40; 
in  muscle,  89;  in  ovarian  follicle  of 
cricket,  39. 

Ammocates,  nephridial  tissue,  353. 

Ammodytes,  pigment  cell,  274. 

Amoeba,  motion  of,  76. 

Amoebocytes,  343. 

Amphibian,  blood  vessels,  159. 

Amphioxus,  blood  channels,  158;  cuticular 
cells,  360. 

Amphitrite,  gills,  328. 

Amphotoky,  474. 

Amplification,  of  epithelial  surfaces,  48-51. 

Ampulla,  of  vertebrate  ear,  212. 

Anal  glands,  of  Carnivora,  400. 

Anaphase,  25. 

Anas,  nerve-endings  on  beak,  204. 

Anisotropic  substance,  83-90. 


Anomia,  attachment,  412. 

Aphrodite,  adhesion,  413. 

Apis,  honey  sac,  291. 

Aplopus,  growth  period,  spermatogonia,  446; 
postsynaptic  spermatogonia,  446 ;  sperma- 
tids,  449 ;  spermatocytes,  448 ;  spermato- 
genesis,  442 ;  summary  of  spermatogenesis, 

449- 

Arrenotoky,  474. 

Arteries,  149. 

Artemia,  parthenogenesis  in,  474. 

Ascaris,  differentiation  of  somatic  cells,  19- 
20;  origin  of  reproductive  cells,  420. 

Aspredo,  carrying  of  eggs,  492. 

Aster,  chromatic,  10. 

Asterias,  maturation  of  reproductive  cells,  426 ; 
maturation  of  female  reproductive  cells, 
461-469;  visual  tissues,  229. 

Asterope,  solencytes,  346. 

Astropecten,  visual  tissues,  229. 

Astroscopus,  electric  tissue,  118. 

Attraction  sphere,  10. 

Auditory  hairs,  216. 

Auditory  tissues,  215;  accessory,  216;  in- 
sects, 216-219;  vertebrates,  220. 

Aurelia,  eye,  231. 

Axial  filament,  428. 

Axis,  of  cell,  10. 

Balancers,  of  insects,  212. 

Barb,  of  feather,  368. 

Barbule,  of  feather,  368. 

Belostoma,  odorous  gland,  406. 

Berce,  grasping  cells,  410. 

Bile  capillaries,  302. 

Bissagenous  granules,  412. 

Bladder,  urinary,  344,  357. 

Blood,  162;  of  Diemyctylis,  165;  of  lobster, 
164;  in  muscle,  82;  of  vertebrates,  165. 

Blood  corpuscles,  red,  172;    white,  165. 

Blood  glands,  150,  167-173;  of  crayfish,  167; 
of  mammals,  168. 

Blood  vessels,  of  Allolobophora,  153;  Amphib- 
ian, 159;  of  Anodonta,  154;  of  Arthro- 
poda,  156-158;  of  Cerebratulus,  151; 
coats  of,  150;  of  Echinoderms,  154;  of 
Imperialis,  158;  of  mammal,  159-161; 
of  Turbellarian  worm,  150. 

Bone,  70;  endochondral,  70;  perichondral,  70. 

Bos,  tendon,  63;    ligamentum  nuchre,  64. 

Branchiomma,  eye,  241. 


5°9 


INDEX 


Branchipus,  muscle  attachment,  66. 

Breathing  apparatus,  320. 

Bufo,  mucous  gland,  393;    poison  or  odorous 

gland,  403. 
Byssus,  of  mollusks,  411 ;    of  ^Enigma,  412. 

Caddis  fly,  nephridia,  344. 

Calcium  phosphate  cells,  of  Mesodon,  356. 

Calla,  root-cap  cell,  10. 

Calliteuthes,  light  organ,  127. 

Catnbarus,  digestive  gland,  298. 

Canaliculi,  bone,  72. 

Cancer  (multipolar  mitosis),  26. 

Capillaries,  149. 

Carchesium,  15. 

Cardiac  muscle,  92. 

Carotid  gland,  304. 

Carpio,  invagination  of  stomach  epithelium, 
51- 

Cartilage,  68;  elastic,  70;  fibrous,  70;  hya- 
line, 69;  of  Loligo,  68-69. 

Cassiopea,  muscles,  88. 

Castania,  supporting  cells,  59. 

Catostomus,  muscle,  81-84;  muscle  histo- 
genesis,  88-91. 

Cavia,  auditory  tissues,  220;    organ  of  Corti, 

222. 

Cell,  i-n;   false,  6. 

Cell -axis,  10. 

Cell-membrane,  1 1 . 

Cell-plate,  25. 

Cell-shape,  12. 

Cell-size,  n. 

Cell- wall,  n. 

Cellulose,  of  Euspongia,  67. 

Cement  substance,  77,  84. 

Centriole,  10. 

Centrosome,  8,  10;   in  nerve  cell,  186. 

Cercaria,  muscle  cell,  103. 

Cerebratulus,    digestive    tissues,    297;     blood 

vessels,    152;    heart  muscle,   93;    muscle, 

98;    unicellular  gland,  52. 
Charybdea,  eye,  230. 
Chauliodus,  light  organ,  137. 
Chief  cells,  of  enteron,  300. 
Chironomus,  auditory  tissues,  218-219. 
Chloragogenic  cells,  343,  355. 
Chloroplast,  9. 

Chordotonal  organs,  216-217. 
Chromaffine  cells,  313. 
Chromatin,  6-7. 
Chromatin,  knot,  8,  181. 
Chromophyllic  substance,  183. 
Chromosome,  23;    valency  of,  24. 
Cilia,  47,  103- 
Circulatory  tissues,  143. 
Claudius,  cells  of,  221. 
Claw,  of  mammal,  385. 
Clitellum,  of  earthworm,  482. 
Cnidocil,  377. 

Coagulation,  of  blood,  163;  in  Crustacea,  164. 
Coccygeal  gland,  304. 
Cochlea,  220. 
Ccelomic  excretion,  353. 


Collecting  fluids,  of  urates,  341. 

Colostrum  corpuscles,  500. 

Columba,  acid  cells,  301 ;    intestine,  296. 

Conductions,  nervous,  174. 

Conjugation,  of  gametes,  418. 

Conjunctiva,  of  alligator,  397. 

Connective    tissues,    56;     higher    forms,    63; 

simple  forms,  61. 
Contractile  vacuole,  339. 
Contraction  stage  of  reproductive  cells,  424. 
Cornea,  of  eye,  226. 
Corpus  cavernosum,  490. 
Corrugation,  49. 
Corti,  organ  of,  221. 
Crotalis,  poison  apparatus,  384. 
Cryptobranchus,  pigment,  275-276. 
Cuticle,  360;    specializations  of,  364. 
Cyclas,  static  tissue,  209;    ciliated  epithelium, 

47- 
Cytoplasm,  6,  8. 

Deiter's  cells,  222. 

Dendrite  (of  nerve  cell),  1 76. 

Desmochondria,  43. 

Desmognathus,  nidamental  tube,  486;    nurse 

cell,  430. 

Diadema,  eye,  233-234;  spine,  377. 
Didelphys,  tonsil,  307-308. 
Differentiation,  of  reproductive  cells,  20;    of 

tissues,  19;   of  somatic  cells,  degrees,  422. 
Digestive   tissues,    297;    Mesodon,   299;    Am- 

phioxus,  299. 
Dicecious  organisms,  419. 
Direct  cell  division,  37. 
Discharge,  of  secreted  material,  5. 
Duct,  53. 

Ductless  gland,  304. 
Duodenum,  of  pigeon,  296. 
Dyads,  dyad  chromosomes,  462. 
Dysatis,  poisoned  spine,  383. 
Dytiscus,  ocellus,  237. 

Ectoderm,  21. 
Ectoplasm,  15. 

Efferent  process,  of  nerve  cell,  176. 
Egg     follicle,     many -layered,     458;      single- 
layered,  455. 
Egg  tubules,  469. 
Electric  connective  tissue,  108. 
Electric  nerve-ending,  107. 
Electric  reticulum,  1 1 1 . 
Electric  rods,  107,  in. 
Electric  tissue,  105;    histogenesis,  113. 
Electrichondria,  107. 
Electroblast,  106. 
Electrolemma,  107. 
Electroplax,  105. 
Eledone,  visual  tissues,  251. 
End-knob,  of  spermatozoon,  429. 
End-organ,  of  nerve  cell,  174. 
End-piece,  of  spermatozoon,  429. 
Endoderm,  21. 
Endoplasm,  15. 
Engelmann's  theory,  muscle,  77. 


INDEX 


Ensatella,  static  power,  210. 

Epeira,  eyes,  239-240. 

Epithelium,  42 ;  developing  muscle,  88 ; 
origin  of,  43 ;  stratified,  45 ;  stratified 
development,  45. 

Equation  division,  of  reproductive  chro- 
mosomes, 426. 

Equatorial  plate,  24. 

Equilibration,  207;  cells,  200. 

Erectile  tissue,  of  man,  490. 

Erinaceus,  spine,  386. 

Erysiphe  (mitosis),  26. 

Erythroblasts,  172. 

Esox,  egg  membranes,  456. 

Eudorina,  14. 

Euspongia,  skeleton,  67. 

Evagination,  50. 

Excretory  cells,  341. 

Excretory  tissues,  339. 

Extinct  animal  series,  13. 

Eye,  general,  224-225;  lubrication  of,  396. 

Fat,  73;  of  chicken,  74;  of  insect,  75;  of 
mammal,  74. 

Feather,  definitive,  370;   down,  370. 

Feathers,  of  birds,  367-382. 

Felis,  egg  follicle,  460;  germ  layers,  21; 
mammary  gland,  499;  oesophagus,  295- 
296;  ovum,  2;  ovum,  growth  of,  469-471; 
placenta,  496;  sebaceous  gland,  394; 
synovial  lubrication,  398;  tactile  nerve- 
endings,  202 ;  tactile  nerve-endings  with 
hair,  206;  wax  glands,  399. 

Female  form  of  cell,  418. 

Fertilization,  467,  468;    polyspermic,  468. 

Fiber,  gastric  glands,  300;   nerve  cells,  181. 

Fibril,  connective  tissue,  56,  64,  66. 

Fibril,  muscle,  77-81;  nerve,  174,  182,  187; 
neuroglia,  197. 

Fin,  of  spermatozoon,  429. 

Flagella,  103. 

Flame-cell,  344. 

Food  vacuoles,  279. 

Forficula,  scent  gland,  405. 

Gadus,  gas  tissues,  335. 

Callus,  adrenal  gland,  315;  gizzard,  287-288; 
nidamental  tissue,  489;  olfactory  tissues, 
259-260;  sebaceous  tissue,  395. 

Gametes,  418. 

Ganglion,  178. 

Gas-secreting  tissues,  333;  of  cod  fish,  335; 
of  paradise  fish,  337;  of  Physophora,  334; 
of  Portuguese  man-of-war,  333;  of  Rhy- 
sophyza,  334. 

Gastric  tissue,  crayfish,  298;  Mesodon,  299; 
muskrat,  300;  pigeon,  300-301. 

Gigantactus,  light  organ,  137. 

Gila  monster,  poison  apparatus,  385. 

Gills,  326;  of  dogfish  embryo,  326;  of  gold- 
fish, 330-332. 

Gizzard,  of  English  sparrow,  287 ;  of  Lum- 
bricus,  287;  of  Seserinus,  288-289. 

Gland,  52;    alveolar,  53;    complex,  54;    sur- 


face,  53;    tubular,   53-54;    types  of,   53; 

unicellular,  52. 

Glomus,  of  nephridia,  341,  343. 
Gonad,  419;    accessory  cells  of,  419;    specific 

cells  of,  419. 
Gonium,  14,  16. 

Grantia,  as  a  cell  colony,  17-18. 
Gryllus  (cricket),  amitosis  in,  39. 
Guanin,  343. 
Gustatory  cell,  262. 
Gustatory  tissues,  258,  261;    of   insects,  263; 

of  Lamperta,  262;  of  Petromyzon,  262. 
Gymnotus,  electric  tissue,  116. 

Haemal  glands,  172. 

Haemocyanine,  163. 

Haemoglobin,  163. 

Hair,  of  mammals,  366. 

Harangus,  rods  and  cones  of  eye,  256. 

Hassall's  bodies,  310. 

Head,  of  spermatozoon,  427. 

Heart,  149. 

Heat,  tissues  which  produce,  141. 

Helix,  nerve  cell,  185;  eye  of,  246;  unicellu- 
lar mucous  gland,  390. 

Hemiptera,  accessory  chromosome,  443. 

Hensen's  cells,  221. 

Hepato-pancreatic  gland,  of  Mesodon,  299. 

Heterotropic  chromosome,  or  heterochro- 
mosome,  444. 

Heterotypic  cell  division,  426. 

Hirudo,  nerve-ending  on  muscle,  194;  neu- 
roglia, 198. 

Homarus,  blood  vessel,  157;  Cardiac  muscle, 
94;  gill,  327;  integument,  362;  Leidig's 
cells,  57;  ligament  cells,  65;  muscle,  85; 
nephridium,  349;  nerve  cell,  1 86;  seminal 
passages,  479. 

Homeotypic  cell  division,  426. 

Homo,  cardiac  muscle,  95;  cartoid  gland, 
317;  coccygeal  gland  of,  316;  optic  nerve 
fiber,  189;  placenta,  498;  respiratory 
tissues,  322;  retina  of,  255. 

Honey  sac,  of  bee,  291. 

Hyacinth,  mitosis  in  root-tip,  26-33. 

Hyalogenesis,  5. 

Hyalogens,  5. 

Hyaloplasm,  7. 

Hydra,  15;  adhesive  cells,  410;  nettle  cells, 
376. 

Hylobius,  adhesive  tissues,  415. 

Hypophysis,  304 ;  of  cat,  306-307 ;  glandular 
lobe,  305;  neural  lobe,  305. 

Iguana,  kidney,  350-352. 

Imperialis,  blood  vessel,  158;  spinning  gland, 
416. 

Impulse,  nerve,  174-176. 

Indirect  cell  division,  23. 

Infundibular  gland,  305. 

Insecta,  accessory  chromosome,  443. 

Integument,  358;  of  birds,  367;  classification 
of,  358;  of  echinoderms,  373;  of  fishes, 
371;  of  lobster,  362;  of  mammals,  364; 


512 


INDEX 


of  man,   364;    of    offensive  devices,   375; 

of  turbellarian  worm,  359. 
Intercalated  disks,  96. 
Intermediate  granule,  83. 
Intermediate  membrane,  83. 
Intermediate   substances  or  tissues,  200. 
Intestine,  of  pigeon,  296. 
Intima,  160. 
Invagination,  50;   relation  to  circulation,  143- 

148. 

Iris,  226. 
Isotropic  substance,  83-90. 

Julus,  scent  gland,  408. 

Karyokinesis,  23. 
Karyoplasm,  6. 
Karyosome,  8. 

Kidney,  341 ;  of  Iguana,  350. 
Krause's  membrane,  83. 

Labium  vestibularis,  223. 

Lacunae,  of  blood,  149;  bone,  71. 

Lamella,  of  byssus  gland,  412. 

Lamina,  of  byssus  gland,  412. 

Lamina  spiralis,  222. 

Lampyris,  light  tissue,  131. 

Lax  connective  tissue,  62. 

Leidig's  cell,  57. 

Leidig's  cells,  connective  tissues,  327. 

Lens,  embryonic  eye,  226;  of  light  organs,  127. 

Lepus,  embryonic  eye,  254;  neuroglia,  197; 
tear  gland  nerves,  195. 

Ligamentum  nuchae,  64. 

Light  tissues,  122;  of  Abraliopsis,  128;  in 
Arthropoda,  131;  of  Calliteuthes,  127; 
Chauliodus,  137;  in  Ccelenterata,  126;  in 
Crustaceans,  129;  in  Ctenophores,  126; 
Gigantactus,  137;  of  Lampyris,  131;  in 
Mollusks,  126;  in  Noctiluca,  125;  of 
Photinus,  133;  development  of,  in  Po- 
richthys,  138-139;  of  Pyrophorus,  132-133; 
quality  of  light,  124;  of  Spinax,  135;  in 
tunicates,  133;  in  worms,  129. 

Limulus,  eye,  239. 

Linin,  7. 

Liver,  of  Cryptobranchus,  302. 

Loligo,  bilateral  symmetry,  22;  cartilage,  68- 
69;  chromatophores,  275-278;  eye  of,  249; 
ink-pigment  tissues,  272;  muscles  in  arm, 
79-80;  muscle  tissue,  99;  nerve  cell,  180; 
shell,  68;  static  cell,  211. 

Lubrication,  387 ;  of  eyes,  396 ;  of  joints. 
398;  list  of  examples,  388;  by  an  oil, 
388;  by  mucin,  387;  by  other  serous 
media,  388. 

Lucif  erase,  122. 

Lumbricus,  calcium  carbonate  glands,  293; 
chloragogenic,  cells,  355;  cuticle,  361; 
gizzard,  287;  mucous  tissue,  391;  ne- 
phridia,  346-348;  nephrostome,  353;  nida- 
mental  tissue,  482;  tactile  nerve-endings, 
201. 

Lungs,  321. 


Lycosa,  poison  gland,  381. 
Lymphatic  nodules,  169. 
Lymph  glands,  168. 

Macropodus,  gas  tissues,  337. 

Macula  acustica,  214. 

Magnolia  (multipolar  mitosis),  26;  nurse  cells, 
430- 

Malapterurus,  electric  tissue,  120. 

Male  form  of  cell,  418. 

Malpighian  tubule,  of  insect,  344. 

Mammal,  adrenal  gland,  315. 

Mammary  glands,  of  cat,  499. 

Mantle  fibrils,  25. 

Marrow,  172. 

Masticatory  tissues,  281;  of  Acanthias,  291; 
of  Apis,  291;  of  Passer,  288;  of  Helix, 
290;  of  Lumbricus,  287;  of  Seserinus, 
288-289. 

Maturation  of  reproductive  cells,  general  ac- 
count, 425;  in  female,  453;  in  male,  436; 
in  a  parthenogenetic  form,  474. 

Media,  160. 

Mesoderm,  2 1 . 

Mesodon,  digestive  cells,  299;  mucous  cell, 
39°- 

Melanoblasts,  271. 

Membrane,  nuclear,  7. 

Membrane  vestibularis,  222. 

Mephitis,  odorous  gland,  401. 

Metaphase,  25. 

Metopus,  pigment  of,  270,  273. 

Microcentrum,  auditory  tissues,  217. 

Microchromosome,  443. 

Micropyle,  of  ovum,  467. 

Microsomes,  3,  8. 

Middle  piece,  of  spermatozoon,  427. 

Mitosis,  23;  multipolar,  26;  without  centro- 
somes,  26;  of  pigment  cells,  272. 

Mochlonyx,  auditory  organ,  217. 

Mole,  adrenal  gland,  315. 

Monads,  monad  chromosomes,  462. 

Monoecious,  419. 

Mormyrus,  electric  tissues,  117. 

Motion,  tissues  of,  76. 

Mucin,  387. 

Mucous  tissue,  of  alligator,  392;  of  Am- 
phibia, 393 ;  of  clam,  389 ;  of  earthworm, 
391 ;  of  mammals,  393 ;  of  snail,  390. 

Mus,  macula  acustica,  214;    tear  gland,  397. 

Muscle,  of  Allolobophora,  99;  of  Ascaris,  98; 
bands  or  stripes,  83;  of  bladder  of  Bos, 
•  100;  cardiac,  92;  cell  of  Cercaria,  102; 
of  Cerebratulus,  98 ;  contraction,  85 ;  de- 
velopment of  smooth,  101 ;  of  Euspongia, 
97;  of  Loligo,  99;  of  Plecypoda,  99; 
smooth,  97;  of  Venus,  100. 

Muscle  cell,  76;   of  Leucosolenia,  102. 

Muscle  cells,  shape,  78-79. 

Muscle  fibril,  76. 

Muscles,  mechanics  of,  79. 

Mya,  mucous  gland,  389. 

My  din,  189. 

Myelocytes,  172. 


INDEX 


513 


Myliobatis,  uterus,  495. 

Myochondria,  76,  82. 

My  old,  cells,  310. 

Myomeres,  88. 

Myotome,  81,  88,  89. 

Mytillus,  byssus,  411;    pigment  in,  271,  274. 

Myzostoma,  egg  follicle,  455. 

Nail,  of  man,  385. 

Nautilus,  eye  of,  253;  olfactory  tissues,  267. 

Nebenkern,  428. 

Necrophorus,  olfactory  tissues,  264. 

Nccturus,  lung,  321. 

Nematocysts,  376. 

Nemertean,  blood  vessels,  151. 

Nephridial  cells,  341. 

Nephridial  tissues,  339-357;  of  coelenterates, 
340 ;  of  Crustacea,  348 ;  of  Eulalia,  345 ; 
of  insects,  344 ;  kinds,  342;  of  lizard,  350- 
352;  of  Lumbricus,  346—348;  origin  of, 
341 ;  of  tapeworm,  344. 

Nephrostome,  343,  352. 

Nereis,  eye,  242. 

Nerve  cell,  processes,  175. 

Nerve  cell  body,  176-181. 

Nerve  cells,  communicatory,  178;  giant,  183- 
184;  grouping  of,  177;  intracellular 
channels,  185;  motor,  178;  origin  of,  176; 
perceptory,  178;  of  Purkinje,  192. 

Nerve-ending,  in  mammal's  cornea,  201 ;  with 
hairs,  205;  of  temperature,  207. 

Nerve-endings,  on  gland  cells,  195;  motor, 
191;  on  muscle,  194. 

Nerve  fiber,  187-188;  covering  of,  188; 
origin  and  growth  of,  187;  regeneration 
of,  1 88. 

Nerve  tissues,  174. 

Nettle  cells,  of  coelenterates,  376. 

Neurite  (of  nerve  cell),  176. 

Neurochondria,  183.  , 

Neuro-fibrils,  174-182. 

Neuroglia,  196-197;    in  visual  tissues,  199. 

Neuromuscular  nerve-ending,  204. 

Neuron,  175. 

Neuron  theory,  1 78. 

Neurons,  grouping  of,  176-177. 

Neurotendinous  nerve-ending,  204. 

Nidamental  tissues,  478;  of  birds,  489;  of 
earthworm,  482 ;  of  fish,  487 ;  of  gastero- 
pod,  483;  of  leech,  480;  of  salamander, 
486;  of  vertebrates,  485. 

Noctiluca,  light  production,  125. 

Normoblasts,  172. 

Notochord,  Opsanus,  60. 

Nourishing  tissues  of  embryo  and  parent,  492. 

Nourishing  tissues  of  embryo,  in  cat,  496;  in 
fish,  500;  in  gull,  499. 

Nourishing  tissues  of  parent,  in  Acanthias, 
492;  in  Myliobatis,  495. 

Nucleus,  6-7 ;  distributed,  6. 

Nurse  cells,  of  developing  ova,  453;  migra- 
tion of,  in  Raja,  438 ;  of  pollen  cells,  430 ; 
of  reproductive  tissues,  420;  of  sperma- 
tozoa, 429;  ovarian,  of  Ameiurus,  456;  of 


2L 


Cambarus,  455;  of  Esox,  456;  of  Felis, 
460;  of  Forficula,  458;  of  mammals,  459; 
of  Myzostoma,  455;  of  Natrix,  458;  of 
Ophryotrocha,  455;  of  Scolia,  457;  of 
Vanesa,  458. 
Nyctiphanes,  light  organ,  129. 

Ocellus,  of  Dytiscus,  237;  of  Perla,  238. 
Octopus,  blood  vessel,  155;   grasping  suckers, 

414;   nerve  fiber,  189;    visual  tissues,  252. 
Odd  chromosomes,  444. 
Odorous  glands,   of  insects,   405;    of  skunk, 

400;   of  stinking  turtle,  402;   of  toad,  403. 
Odors,  attractive  and  offensive,  400. 
(Esophagus,  of  cat,  295-296;  of  squid,  295. 
Olfactory  bulb,  193. 
Olfactory  cell,  stimulation  of,  261. 
Olfactory  tissues,  258;  of  mollusks,  266-288. 
Ontogeny,  19. 
Oocyte,  of  first  order,  461. 
Oogonium,  461. 
Oosperm,  419,  461. 
Ophiura,  photogenesis,  123. 
Ophryotrocha,  egg  follicle,  455. 
Opsanus,    notochord,    60;     yolk    membrane, 

500. 

Orders  of  colonization,  14. 
Organ,  15,  19. 
Organization,  19. 
Osphradium,  266-267. 
Osteoblasts,  71. 
Osteoclasts,  73. 
Ova,  follicle  cells,  454;  food  acquisition,  453; 

growth,  and  maturation  of,  453. 
Ovary,  419. 

Ovis,  developing  stratified  epithelium,  46. 
Ovum,  growth  of,  in  a  mammal,  469-473. 

PalcRmon,  eye,  235. 

Palamonetes,  static  hair,  208;  tactile  hair,  205. 

Pancreatic  tissues,  300. 

Pandorina,  14. 

Papilla,  of  feather,  368. 

Parablastic  nuclei,  in  fish,  501. 

Paraganglionic  bodies,  313. 

Paramcecium,  food  vacuoles,  284. 

Paraplasm,  3. 

Parasynapsis   of   reproduction   chromosomes, 

425- 

Parathyroid  gland,  304;   of  cat,  312. 
Parthenogenesis,  474. 
Pecten,  eye  of,  246-249. 
Perceptory  organs,  of  nerve  cells,  174. 
Pericardial,  glands,  343,  354. 
Pericheeta,  nephrostome,  352. 
Periphery,  of  circulation,  149. 
Periplanata,  eye,  234. 
Peyer's  patches,  308. 
Pholas,  light  tissue,  126. 
Pholcus,  yolk  body,  461. 
Photinus,  light  tissue,  133. 
Photochondria,  125. 
Photo  plasm,  125. 
Phylodoce,  eye  of,  243. 


INDEX 


Phylogeny,  13. 

Physalia,  gas  tissues,  333. 

Physophora,  gas  tissues,  334. 

Pieris,  spinning  gland,  417. 

Pigment,  269;  of  Ammodytes,  274-275;  of 
Cryptobranchus,  274,  276;  diffused,  269; 
of  Loligo  (chromatophores),  275-278;  of 
Loligo  (ink),  272;  of  Metopus,  274;  in 
nerve  cell,  185,  186;  of  salamander,  271. 

Pigment  mantle,  light  organs,  123. 

Pigment  segregated,  270. 

Pinna,  215,  223. 

Pipa,  carrying  of  eggs,  492- 

Pisicola,  nidamental  cells,  480. 

Pituitary  body,  304. 

Placenta,  of  cat,  496 ;   of  man,  498. 

Planaria,  eye,  231-232. 

Planocera,  integument,  359. 

Plasmosomes,  7. 

Plastids,  9. 

Poison  gland,  of  Arachnids,  381 ;  of  catfish, 
384;  of  ground -hornet,  379;  of  rattle- 
snake, 384 ;  of  toad,  403 ;  of  vertebrates, 

383- 

Poison  hairs,  of  saddle-back  larva,  379. 
Polar  body,  first,  462 ;  second,  462. 
Polarity  (cell),  10. 
Pollen  cells,  of  Magnolia,  431. 
Pollen  formation,  Magnolia,  430-436. 
Pollen  mother  cells,  431. 
Pollen  sac,  431. 

Polygordius,  nephrostomes,  352. 
Pontobdella,  sucking  disk,  413. 
Porichthys,  development  of  light  organs,  138- 

139- 

Postreduction,  426. 

Preformation,  20. 

Premyelocytes,  172. 

Pre-oocyte,  of  cat,  471. 

Prereduction,  426. 

Pre-spermatogonia,  of  selachian  fish,  437. 

Primordial  egg  cells,  469. 

Prophase,  25. 

Prostate  gland,  of  mammal,  478. 

Proteids,  3. 

Protoplasm,  i. 

Protoplast,  i. 

Pseudocyst,  6. 

Pseudopleuronectes,  giant  nerve  cell,  183;  in- 
fundibular gland,  305. 

Pseudostratified  epithelium,  46. 

Pterophryne,  7;  giant  nerve  cell,  184;  nida- 
mental tissue,  487. 

Pyrosoma,  light  tissue,  133. 

Rachis,  of  feather,  368. 

Radula,  of  Helix,  290. 

Raja,  ampulla,  212;  electric  tissue,  108;  histo- 
genesis  of  electric  tissue,  113;  nidamental 
tissue,  487;  renal  bodies,  314;  spermato- 
genesis  of,  436-443 ;  thyroid  gland,  311-312. 

Rana,  renal  nerve-endings,  196. 

Reducing  division,  of  reproductive  chromo- 
somes, 425. 


Reduction,  of  pollen  cell,  Magnolia,  433. 

Reduction,  of  reproductive  cells,  425. 

Reduction  divisions,  in  mouse,  472-474. 

Reflecting  tissue,  of  light  organs,  123. 

Remora,  adhering  organ,  414. 

Renal  bodies,  313. 

Reproduction,  outline,  418. 

Reproductive  cells,  19,  418;  development  of 
female  form,  453;  development  of  male 
form,  423;  their  differentiation,  19;  origin 
of,  in  the  individual,  420;  sexual  and 
asexual,  418. 

Respiratory  tissues,  319;  of  Amphitrite,  328; 
of  Crustacea,  324;  of  dogfish,  326;  of 
fishes,  330;  of  insects,  324;  of  man,  322; 
of  mollusks,  322 ;  of  adult  salamander,  321 ; 
of  snail,  323;  of  Sycotypus,  330. 

Reticular  theory,  of  protoplasm,  3. 

Retina,  226;  diagram  of  human,  257. 

Retinulae,  224. 

Rhabdites,  375. 

Rhabdome,  225. 

Rhopalonema,  static  tissues,  209. 

Rhyzophysa,  gas  tissues,  334. 

Rod,  visual,  225. 

Sacculus,  220. 

SagMa,  stratified  epithelium,  45. 

Sarcoblast,  90. 

Sarcolemma,  78-82. 

Sarcomere,  77. 

Sarcoplasm,  76. 

Sarcous  element,  77. 

Scale,  of  fishes,  371. 

Scent  gland,  of  bugs,  406 ;  of  centipedes,  408 ; 
of  earwig,  405 ;  of  Julus,  408 ;  of  skunk, 
401 ;  of  toad,  403;  of  turtle,  402. 

Schilbeodes,  poison  apparatus,  383. 

Scolia,  digestive  tissue,  298 ;   poison  apparatus, 

379- 

Scolophores,  or  auditory  cells,  219. 
Sebaceous   tissues,    394 ;     of   birds,    395 ;     of 

mammals,  394. 
Secretion,  5. 

Sepia,  visual  tissue,  251. 
Serous  glands,  of  bat,  301. 
Sertoli  cell,  423,  430. 
Seserinus,  gizzard  and  teeth,  289. 
Sex,  determination  of,  422,  442,  450;    tables 

showing,  4SI-4S2. 
Sibine,  poison  hairs,  379. 
Smooth  muscle,  97. 
Solen,  visual  tissue,  227-228. 
Spelerpes,  10. 

Sperm  cells,  of  Volvox,  17. 
Sperm  columns,  423. 
Spermatic  lobule,  423. 
Spermatid,  425. 
Spermatocyte,  of  first  order,  424;    of  second 

order,  424. 

Spermatogenesis,  season  of,  424;  of  skate,  436. 
Spermatogonium,  424;    growth  of    (general), 

429. 
Spermatophoral  gland,  479. 


INDEX 


515 


Spermatophores,  of  cephalopods,  480. 

Spermatotheca,  479;    of  lobster,  479. 

Spermatozoa,  425;  types  of,  427. 

Spider,  eyes,  239. 

Spinax,  light  organs,  134. 

Spindle,  25. 

Spindle  fibrils,  25. 

Spine,   of  Echinoderms,   377;    of  porcupine, 

385- 

Spinning  gland,  of  Lepidoptera,  416. 

Spinning  tissues,  409. 

Spireme,  23. 

Spleen,  170. 

Sponge,  muscle  cell  on  water  pore,  102;  sup- 
porting cells,  61. 

Spongioplasm,  3. 

Spores  (reproductive  cells),  418. 

Static  power,  by  gravity,  207,  212;  spatial, 
207,  212. 

Static  tissues,  207;  of  Cephalopoda,  211;  of 
Crustacea,  208;  of  Cyclas,  2 1  o ;  of  Insecta, 
21 1 ;  of  medusae,  209 ;  of  Vertebrata,  212. 

Stigmata,  227. 

Stimulus,  nerve,  174,  177. 

Sting,  of  ground-hornet,  379. 

Stratification,  of  epithelium,  364. 

Strombus,  eye  of,  244-245. 

Sulcus  spiralis,  222. 

Supporting  tissue,  56,  58;   of  chestnut,  58. 

Supporting  tissues,  higher  forms,  67 ;  simple 
forms,  57. 

Sus,  nerve-ending  in  snout,  201 . 

Sweat  glands,  of  mammals,  398. 

Sycotypus,  cardiac  muscle,  93;  nephridial 
tissue,  346;  neuroglia,  198;  nidamental 
tissue,  483;  olfactory  tissue,  266. 

Synapsis,  461 ;  469. 

Synizesis,  461 ;  of  reproductive  chromosomes, 
425 ;  of  reproductive  cells,  424. 

Synovial  fluid,  398. 

Synovial  membrane,  of  cat,  398. 

Tactile  cells,  200. 

Tactile  nerve-endings,  with  capsules,  202-204 ; 

in  conjunctiva  of  man,  203;   simple  forms, 

201 ;  of  Herbst,  204. 
Tail,  of  spermatozoa,  429. 
Taxonomic  series,  13. 
Tear  gland,  of  mouse,  397. 
Technic,  502;  outline  of,  502. 
Teeth,  of  Acanthias,  291-294;    of  Seserinus, 

289. 
Telosynapsis,    of  reproductive   chromosomes, 

425- 

Tendon,  cow,  63. 
Terminal  bars,  43,  360. 
Terrapene,  stink  gland,  402 
Testis,  419. 
Tetrads,  462. 


Tetramitus,  6. 

Tetronarce,  chromatin  knot,  8 ;  histogenesis  of 

electric  tissue,  114;  nerve  cell,  180. 
Thallassophryne,  poison  gland,  384. 
Thelytoky,  474. 
Thymic  lymphocytes,  310. 
Thymus  gland,  304;  of  cat,  309;  of  frog,  310; 

of  man,  310. 

Thyroid  gland,  304;    of  fish,  311. 
Tonsil,  304;  of  opossum,  307. 
Touch,  tissues  of,  200. 
Tracheal  tube,  of  insect,  325. 
Transportation,  of  spermatozoon,  427. 
Trichocysts,  of  Infusoria,  375. 
Triodopsis,  lung,  323. 
Trophospongia,  12. 
Tropidonotus,  olfactory  cells,  261. 
Turbellarian,  blood  vessels,  150. 
Tympanum,  215,  223. 

Umbilical  cord,  sheep,  61. 
Umbilicus,  of  feather,  368. 
Unto,  mitosis  of,  33-37;  nephridial  tissues, 

354;    static  tissue,  210. 
Ureter,  344. 
Urethra,  344. 
Utriculus,  of  vertebrate  ear,  220. 

Vacuole,  9;  food,  279. 

Vampyrella,  pigment  of,  273. 

Vanadis,  eye  of,  243. 

Vas  deferens,  437. 

Vaso-formative  cells,  162. 

Veins,  149. 

Venus,  100. 

Vespertillo,  mucous  glands,  294-295. 

Visual  cells  and  tissues,  224. 

Visual  tissues,  of  arthropods,  234-240;  tetra- 
branch  cephalopods,  253;  of  ccelenterates, 
230-231;  of  echinoderms,  228,  233;  of 
mollusks,  244-253;  of  plecypod  mollusks, 
227;  of  planarians,  231 ;  of  Protozoa,  226; 
vertebrates,  253-257;  of  worms,  241. 

Vital  law,  4. 

Vitelline  membrane,  467. 

Volvox,  14,  1 6,  17. 

Vorticella,  contractile  stalk,  102. 

Wandering  cells,  343. 
Wood  cells,  60. 

Yolk  membranes,  of  bird,  500 ;    of  fish,  501 . 
Yolk  nucleus,  of  Felis,  470;   of  Limulus,  460; 
of  Lophius,  460 ;  of  Pholcus,  460. 

Zona  radiata,  456. 
Zygote,  461. 


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